Wireless communication terminal, base station device, resource allocation method

ABSTRACT

A radio communication terminal that increases the ACK/NACK resource utilization efficiency while preventing ACK/NACK collision, and that causes no unnecessary reduction of the PUSCH band in a system that transmits E-PDCCH control information. The radio communication terminal adopts a configuration including a receiving section that receives a control signal including an ACK/NACK index via an enhanced physical downlink control channel (E-PDCCH) transmitted using one configuration from among one or a plurality of configuration candidates, a control section that selects a resource to be used for an ACK/NACK signal of downlink data from among specified resources specified beforehand based on E-PDCCH configuration information used for transmission or reception of the E-PDCCH and the ACK/NACK index, and a transmitting section that transmits the ACK/NACK signal using the selected specified resource.

BACKGROUND Technical Field

The present invention relates to a radio communication terminal, a basestation apparatus, and a resource allocation method.

Description of the Related Art

The 3GPP (3rd Generation Partnership Project Radio Access Network) hasestablished LTE (Long Term Evolution) Rel.8 (Release 8) and the enhancedversion of LTE, which is LTE Rel.10 (LTE-Advanced). In these standards,a base station, and a radio communication terminal (also called “UE(User Equipment)” and referred to below as a terminal) transmit controlinformation for transmitting and receiving data using a downlink PDCCH(physical downlink control channel) (refer to Non-Patent Literatures 1to 3). FIG. 1 shows the subframe configuration of the downlink. In thesubframes, the PDCCH that transmits a control signal and the PDSCH(physical downlink shared channel) that transmits a data signal aretime-division multiplexed. The terminal first decodes the controlinformation transmitted to the terminal through the PDCCH and obtainsinformation regarding a frequency allocation required for data receptionon the downlink, and adaptive control, for example. The terminal thendecodes data for the terminal that is included in the PDSCH, based onthe control information. If control information that permits datatransmission on the uplink is included in the PDCCH, the terminaltransmits data on the PUSCH (physical uplink shared channel) of theuplink, based on the control information.

In order to transmit and receive data on the downlink, an HARQ (hybridautomatic request) combining error correction decoding and an automaticretransmission request has been introduced. After performing errorcorrection decoding, the terminal judges whether or not the data iscorrectly decoded, based on a CRC (cyclic redundancy checksum) appendedto the data. If the data is correctly decoded, the terminal feeds backan ACK to the base station. If, however, the data is not correctlydecoded, the terminal feeds back a NACK to the base station, promptingretransmission of the data in which an error is detected. The feedbackof ACK/NACK (acknowledge response; hereinafter referred to as “A/N”) istransmitted on the uplink. If data is not assigned to the PUSCH at thetime of A/N transmission, A/N is transmitted on the PUCCH (physicaluplink control channel). If, however, data is assigned to the PUSCH atthe time of A/N transmission, A/N is transmitted on either the PUCCH orthe PUSCH. When this is done, the base station instructs the terminalbeforehand as to whether transmission is to be done on the PUCCH or thePUSCH. FIG. 2 shows the uplink subframe configuration that includes thePUSCH and the PUCCH.

If A/N is transmitted on the PUCCH, there are situations to be handleddifferently. For example, if the A/N transmission overlaps with thefeedback of CSI (channel state information) periodically transmitted onthe uplink, the PUCCH formats 2a/2b are used. On the downlink, ifcarrier aggregation, in which transmission is performed using aplurality of carriers that are bundled together, is set to ON, and alsothe number of carriers is at least three, the PUCCH format 3 is used.However, regardless of whether carrier aggregation is OFF or ON, if thenumber of carriers is two or fewer and there is no control informationother than A/N and other than an uplink scheduling request, even if thenumber of carriers does not exceed two, the PUCCH formats 1a/1b areused. In considering that downlink data is transmitted more frequentlythan uplink data, and also considering that the period of CSI feedbackis not more frequent than the period of downlink data assignment, A/N ismost often transmitted by the PUCCH formats 1a/1b. The followingdescription will focus on the PUCCH formats 1a/1b.

FIG. 3 shows the slot configuration of the PUCCH formats 1a/1b. The A/Nsignals transmitted by a plurality of terminals are distributed by theWalsh sequence having a length-4 sequence and a DFT (discrete Fouriertransform) sequence having a length-3 sequence and are code multiplexedand received at the base station. In FIG. 3, (W₀, W₁ , W₂, W₃) and (F₀,F₁, F₂) represent, respectively, the above-noted Walsh sequence and DFTsequence. At the terminal, a signal representing either ACK or NACKfirst undergoes primary spreading to frequency components correspondingto one SC-FDMA symbols by a ZAC (zero auto-correlation) sequence (with asubcarrier having a length-12 sequence) in the frequency domain. Thatis, a ZAC sequence having a sequence length of 12 is multiplied by anA/N signal component represented by a complex number. Then, the A/Nsignal after primary spreading and the ZAC sequence as a referencesignal undergo secondary spreading by a Walsh sequence (W₀ to W₃ of alength-4 sequence, also called a Walsh code sequence) and a DFT sequence(F₀ to F₂ of a length-3 sequence). That is, each component of a signalhaving a length-12 sequence (an A/N signal after primary spreading or aZAC sequence (reference signal sequence)) is multiplied by eachcomponent of an orthogonal sequence (for example, a Walsh sequence or aDFT sequence). Additionally, the signal after secondary spreading istransformed by an IFFT (inverse fast Fourier transform) to a length-12sequence (subcarrier) signal in the time domain. Then, a CP (cyclicprefix) is added to each signal after the IFFT, thereby forming aone-slot signal made up of seven SC-FDMA symbols.

A/N signals from different terminals are spread using ZAC sequencescorresponding to different cyclic shift indexes or orthogonal codesequences corresponding to different orthogonal cover indexes (OCindexes). The orthogonal code sequence is a set of a Walsh sequence anda DFT sequence. The orthogonal code sequence is also called a block-wisespreading code sequence. Therefore, by using the conventionaldespreading and correlation processing, the base station can demultiplexthe plurality of A/N signals that have been code multiplexed and cyclicshift multiplexed. Because there is a limit to the number of A/N signalsthat can be code multiplexed and cyclic shift multiplexed per frequencyresource block (RB), if the number of terminals becomes large, frequencymultiplexing is performed using different RBs. In the following, thecode-RB resource in which A/N is transmitted will be called the A/Nresource. The A/N resource number is determined by the number of the RBin which A/N is transmitted and the code number and cyclic shift valuein the RB. Because multiplexing by cyclic shifting of the ZAC sequencecan be treated as a type of code multiplexing, there will be cases inwhich orthogonal code and cyclic shift will be collectively called codein the following description.

In LTE, in order to reduce interference from other cells on the PUCCH,the ZAC sequence to be used is determined based on the cell ID. Becausethe mutual correlation between different ZAC sequences is small, byusing different ZAC sequences between different cells, the interferencecan be reduced. Also, in the same manner, sequence hopping and cyclicshift hopping based on the cell ID has been introduced. With thesehoppings, shifting is done cyclically in units of SC-FDMA symbols, usinga cyclic shift hopping pattern determined based on the cell ID, whilemutual correlation on the cyclic shift axis and orthogonal code axis aremaintained. Doing this enables randomization of combinations of A/Nsignals that are strongly interfered by other cells, while the mutualorthogonal relationship between A/N signals are maintained within acell, and also enables removal of continuous strong interference to onlysome of the terminals from other cells.

In the description to follow, the description will be of the case inwhich a ZAC sequence is used for primary spreading, and a block-wisespreading code sequence is used for secondary spreading. However, forthe primary spreading, rather than using a ZAC sequence, sequences thatare mutually separable by mutually different cyclic shift values may beused. For example, a GCL (general chirp-like) sequence, a CAZAC(constant amplitude zero auto correlation) sequence, a ZC (Zadoff-Chu)sequence, a PN sequence such as an M sequence or an orthogonal Gold codesequence, or a computer-generated random sequence having sharpautocorrelation characteristics may be used for the primary spreading.As long as the sequence can be treated as being mutually orthogonal orsubstantially mutually orthogonal, any sequence can be used as ablock-wise spreading code sequence for the secondary spreading. Forexample, a Walsh sequence or a Fourier sequence or the like can be usedas a block-wise spreading code sequence for the secondary spreading.

In LTE, as a method of allocating different A/N resources to differentterminals, allocation is used that is based on control informationmapping results of the PDCCH. That is, using the fact that PDCCH controlinformation is not mapped onto the same resources between a plurality ofterminals, a one-to-one correspondence is established between the PDCCHresources and the PUCCH formats 1a/1b A/N resources (hereinafterdescribed simply as A/N resources). This will be described in detailbelow.

The PDCCH is made up of one or more L1/L2 CCHs (L1/L2 control channels).Each L1/L2 CCH is made up of one or more CCEs (control channelelements). That is, a CCE is the basic unit of mapping controlinformation onto a PDCCH. Also, when one L1/L2 CCH is made up of aplurality (2, 4, or 8) of CCEs, a plurality of continuous CCEs with aCCE having an even-numbered index as the origin is allocated to thatL1/L2 CCH. The base station, in accordance with the number of CCEsnecessary for notification of control information to the subjectterminal to which resources are to be allocated, allocates an L1/L2 CCHto the terminal to which the resources are to be allocated. The basestation then maps the control information onto the physical resourcescorresponding to the CCE of that L1/L2 CCH. In this case, there is aone-to-one correspondence between each CCE and A/N resource. Therefore,a terminal that has received an L1/L2 CCH identifies the A/N resourcescorresponding to the CCEs making up that L1/L2 CCH, and uses thoseresources (that is, codes and frequencies) to transmit the A/N signal tothe base station. However, in the case of the L1/L2 CCH occupying aplurality of continuous CCEs, the terminal uses an A/N resourcecorresponding to the CCE having the smallest index of a plurality ofPUCCH constituent resources corresponding to a plurality of CCEs (thatis, the A/N resource that has been associated with the CCE having a CCEindex that is even number) to transmit the A/N signal to the basestation. Specifically, the A/N resource number n_(PUCCH) is establishedby the following equation (equation 1) (e.g., see Non-Patent Literature3).[1]n _(PUCCH) =N+n _(CCE)   (Equation 1)

In this case, the above-noted A/N resource number n_(PUCCH) is theabove-described A/N resource number, N is the A/N resource offset valuegiven in common within the cell, and n_(CCE) is the number of the CCEonto which the PDCCH is mapped. According to equation 1, it can be seenthat, in accordance with the range that can be taken by n_(CEE), an A/Nresource within a certain range can be used. In the following, the A/Nthat determines the resources dependent upon the control informationscheduling of the PDCCH in this manner will be noted as D-A/N (dynamicA/N (dynamic ACK/NACK)).

As described above, the A/N resources include frequency resources inaddition to code resources. Because the PUCCH and the PUSCH use the samefrequency band in the uplink, there is a tradeoff between the region ofthe PUCCH that includes the D-A/N and the bandwidth of the PUSCH.

CITATION LIST Non-Patent Literature

NPL 1

3GPP TS 36.211 V10.4.0, “Physical Channels and Modulation (Release 10),”December 2011

NPL 2

3GPP TS 36.212 V10.4.0, “Multiplexing and channel coding (Release 10),”December 2011

NPL 3

3GPP TS 36.213 V10.4.0, “Physical layer procedures (Release 10),”December 2011

BRIEF SUMMARY Technical Problem

Because the PDCCH has a limited region for the assignment of controlinformation, the number of terminals and amount of control informationthat can be assigned simultaneously is limited. Also, the PDCCH issupposed to be received in accordance with cell-specific parameters.Because the PDCCH is dependent on the cell-specific parameters, there isa problem in that the PDCCH is not suitable for CoMP (coordinatedmultipoint) operation in which there is coordination between a pluralityof cells or an HetNet (heterogeneous network) in which pico basestations are disposed and operated within a cell formed by a macro basestation. In this respect, the adoption of an E-PDCCH (enhanced PDCCH(enhanced downlink control channel)) as a new control channel differentfrom the PDCCH, is under discussion in Rel.11.

Adoption of the E-PDCCH enables an increase in the region to whichcontrol information is allocated. Additionally, the E-PDCCH has theadvantage of enabling flexible control information allocation that isnot restricted by the settings in units of cells. For this reason,adoption of the E-PDCCH is expected to enable operation suitable inparticular for CoMP, in which coordination is made between cells, andfor HetNet, in which inter-cell interference control is important.

When the E-PDCCH is adopted, however, unless some measure is taken, itis assumed that collision can occur in the uplink A/N between a terminalcontrolled by the E-PDCCH control information and a terminal controlledby the PDCCH control information. Alternatively, a problem can beassumed in which the PUSCH band is reduced, if excessive A/N resourcesare reserved wastefully to prevent A/N collision.

An object of the present invention is to provide a radio communicationterminal, a base station apparatus, and a resource allocation methodthat, while preventing A/N collision, increase the A/N resourceutilization efficiency and that cause no unnecessary reduction of thePUSCH band in a system that transmits E-PDCCH control information.

Solution to Problem

A radio communication terminal according to an aspect of the presentinvention includes: a receiving section that receives a control signalincluding an ACK/NACK index via an enhanced physical downlink controlchannel (E-PDCCH) transmitted using one configuration from among one ora plurality of configuration candidates; a control section that selectsa resource to be used for an ACK/NACK signal of downlink data from amongspecified resources specified beforehand, based on E-PDCCH configurationinformation used for transmission or reception of the E-PDCCH and theACK/NACK index; and a transmitting section that transmits the ACK/NACKsignal using the selected specified resource.

A base station apparatus according to an aspect of the present inventionincludes: a control section that determines a resource for transmittingan ACK/NACK signal in response to downlink data from a radiocommunication terminal from among specified resources specifiedbeforehand, based on a configuration used for transmission of an E-PDCCHout of one or a plurality of E-PDCCH configurations indicated beforehandto the radio communication terminal and an ACK/NACK index included in acontrol signal; and a transmitting section that transmits the controlsignal including the ACK/NACK index indicating the determination resultof the control section via the E-PDCCH using the configurationcorresponding to the determined specified resource.

A resource allocation method according to an aspect of the presentinvention includes: receiving a control signal including an ACK/NACKindex via an enhanced physical downlink control channel (E-PDCCH); andselecting one of specified resource candidates specified beforehand fromamong a plurality of ACK/NACK resources separated from each other infrequency and code regions, based on the ACK/NACK index and aconfiguration of the E-PDCCH.

Advantageous Effects of Invention

According to the present invention, in the case where controlinformation is transmitted using the enhanced physical downlink controlchannel and the physical downlink control channel, it is possible toincrease the A/N resource utilization efficiency and to avoid anunnecessary reduction of the PUSCH band while avoiding collision betweenA/N signals for downlink data.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a drawing showing the subframe configuration of the downlink;

FIG. 2 is a drawing showing the subframe configuration of the uplink;

FIG. 3 is a drawing describing the method of spreading of the A/N signalin the PUCCH formats 1a/1b;

FIGS. 4A to 4C are drawings showing an example of the subframeconfiguration of the downlink at the time of E-PDCCH transmission;

FIG. 5 is a drawing showing the system configuration when E-PDCCH isadopted;

FIG. 6 is a drawing showing examples in which the D-A/N region for aPDCCH terminal and the D-A/N region for an E-PDCCH terminal are set;

FIG. 7 is a drawing showing examples in which different A/N resourcesare allocated to four E-PDCCH terminals, respectively;

FIG. 8 is a block diagram showing the main parts of a base station ofEmbodiment 1;

FIG. 9 is a block diagram showing the details of the base station ofEmbodiment 1;

FIG. 10 is a block diagram showing the main parts of a terminal inEmbodiment 1;

FIG. 11 is a block diagram showing the details of the terminal inEmbodiment 1;

FIG. 12 is a drawing showing an example of E-PDCCH scheduling inEmbodiment 1;

FIGS. 13A and 13B are drawings describing A/N resources of an E-PDCCHterminal switched based on an ARI according to Embodiment 1;

FIGS. 14A and 14B are drawings showing an A/N resource candidate settingrange corresponding to the E-PDCCH configuration according to Embodiment1;

FIGS. 15A and 15B are drawings showing the subframe configuration of thedownlink according to Embodiment 2;

FIG. 16 is a drawing showing an example of E-PDCCH scheduling inEmbodiment 2;

FIG. 17 is a drawing showing A/N resources identified based on a PRB setwhereby E-PDCCH is transmitted according to Embodiment 2;

FIG. 18 is a drawing showing the subframe configuration of the downlinkaccording to Embodiment 2;

FIG. 19 is a drawing showing the subframe configuration of the downlinkaccording to Embodiment 3;

FIG. 20 is a drawing showing an example of E-PDCCH scheduling inEmbodiment 3;

FIG. 21 is a drawing showing A/N resources identified based on a searchspace whereby E-PDCCH is transmitted according to Embodiment 3;

FIG. 22 is a drawing showing the subframe configuration of the downlinkaccording to Embodiment 4;

FIG. 23 is a drawing showing an example of E-PDCCH scheduling inEmbodiment 4;

FIG. 24 is a drawing showing A/N resources identified based on atransmission mode used for transmission of an E-PDCCH according toEmbodiment 4;

FIG. 25 is a drawing showing the subframe configuration of the downlinkaccording to Embodiment 5;

FIG. 26 is a drawing showing the subframe configuration of the downlinkaccording to Embodiment 6;

FIG. 27 is a drawing showing the other subframe configuration of thedownlink according to Embodiment 6;

FIG. 28 is a drawing showing the other subframe configuration of thedownlink according to Embodiment 6;

FIG. 29 is a drawing showing a communication system in Embodiment 7; and

FIG. 30 is a drawing showing an example of E-PDCCH scheduling inEmbodiment 7.

DETAILED DESCRIPTION

Embodiments of the present invention will be described in detail belowwith references made to the drawings.

Embodiment 1

<Background of Obtaining an Embodiment of the Present Invention>

First, before describing the specific configuration and operation ofEmbodiment 1, one method that the inventors of the present inventionhave noticed as a method for allocating A/N resources in the case ofadopting the E-PDCCH will be described.

FIGS. 4A to 4C show an example of the downlink subframe at the time ofE-PDCCH transmission. FIG. 5 shows the system configuration when E-PDCCHis adopted.

An E-PDCCH has some or all of the following features.

(1) Unlike a PDCCH transmitted using resources common to all terminals,the E-PDCCH is transmitted using a frequency resource block allocated toeach terminal.

(2) Unlike the PDCCH demodulated using a reference signal common to allterminals in a cell, the E-PDCCH is demodulated using aterminal-specific reference signal given to each terminal.

(3) Unlike the PDCCH scrambled using a scramble code common to allterminals in the cell, the E-PDCCH is scrambled using a scramble codegiven to each terminal.

(4) Whether or not to transmit the E-PDCCH can be changed by a setting.

As shown in FIGS. 4A to 4C, unlike the PDCCH transmitted using resourcescommon to all terminals, a frequency resource block (PRB) is set foreach terminal and the E-PDCCH is transmitted using the PRB. In theexample of FIGS. 4A to 4C, PRB numbers 2, 4, . . . , 24, 26 are set asE-PDCCHs. Each E-PDCCH is composed of one or a plurality of resourceeCCEs (enhanced control channel elements). The relationship between aneCCE number and a PRB number is not defined yet, but in consideration ofthe fact that the E-PDCCH is set for each terminal, the followingrelationship can be assumed.

(1) Numbering such that different eCCE numbers are assigned to all PRBsin entire system band (FIG. 4A)

(2) Numbering such that all eCCE numbers are different in one or aplurality of PRB sets with which E-PDCCHs set for each terminal aretransmitted (FIG. 4B)

(3) Numbering such that all eCCE numbers are different in each PRBirrespective of set PRBs (FIG. 4C)

Furthermore, as shown in FIG. 5, in a communication system adopting theE-PDCCH, it is expected that PDCCH terminals and E-PDCCH terminals aremixed within one cell (the E-PDCCH terminals in FIG. 5 being shown inblack). In this case, a PDCCH terminal is a terminal that receives PDCCHcontrol information for controlling its communication, and an E-PDCCHterminal is a terminal that receives E-PDCCH control information forcontrolling its communication.

Therefore, adopting the E-PDCCH enables flexible control informationassignment without restriction by the setting in units of cells inaddition to increasing the region of the control information. Forexample, a plurality of E-PDCCHs with different settings within a cellcan be used or E-PDCCHs with the same setting between cells can be used.For this reason, adoption of the E-PDCCH is expected to enable operationsuitable in particular for CoMP, in which coordination is made betweencells, and for HetNet, in which inter-cell interference control isimportant.

On the other hand, no method has been defined so far for determining A/Nfeedback resources in response to the PDSCH to which E-PDCCH isallocated as control information.

The simplest method is to establish the A/N resource number as shown inthe following equation 2, for example, using the E-PDCCH, similar to thePDCCH.[2]n _(PUCCH) ^(E-PDCCH) =f(N _(e) , n _(eCCE))   (Equation 2)

In the above, n_(PUCCH) ^(E-PDCCH) is the resource number with which theE-PDCCH terminal transmits the A/N, N_(e) is the A/N resource offsetvalue, and n_(eCCE) is the number of the eCCE to which the E-PDCCH ismapped. Also, N_(e) is the D-A/N resource offset parameter, and this maybe a cell-specific value, or a value given independently for eachterminal individually. The function f(a, b) is f(a, b)=a+b, for example.

This method has the advantage of not requiring notification of the A/Nresource to each terminal, and also having no possibility of A/Ncollision between E-PDCCH terminals. On the other hand, the method hasthe disadvantage that A/Ns of the E-PDCCH terminal are distributed overa wide range and A/Ns collide with each other between a plurality ofterminals, generating an allocation block. FIG. 6 shows this situation.

FIG. 6 illustrates an example of a case where A/N resources for thePDCCH terminal and A/N resources for the E-PDCCH terminal are set, fourA/N resources each. Suppose the A/N resources for the PDCCH terminal aredetermined according to conventional equation 1. On the other hand,suppose the A/N resources for the E-PDCCH terminal are determinedaccording to equation 2.

First, determining A/N resources of the E-PDCCH terminal using the eCCEnumber results in a problem that A/N resources are distributed over awide range. The degree of distribution differs depending on the range ofvalues that can be taken by the eCCE number and equation 2. For example,when eCCE numbering as shown in FIG. 4A is done, the degree ofdistribution of A/N resources becomes very large, which may reduce thebandwidth in which the PUSCH should originally be transmitted. This maylead to deterioration of the uplink throughput.

Moreover, there is also a problem that A/N resources collide with eachother. FIG. 6 shows a situation in which A/N collision occurs betweenthe PDCCH terminal and the E-PDCCH terminal. Actually, a plurality ofE-PDCCHs may be set within a cell and A/N collision may occur alsobetween different E-PDCCHs in this case. Collision deteriorates the A/Nquality significantly and is never acceptable, and therefore whencollision occurs in A/N resources between a plurality of terminals,allocation needs to be given up. On the other hand, it is possible toattempt to avoid an allocation block by rescheduling PDCCH or E-PDCCHamong a plurality of terminals, which would, however, require schedulingof both downlink control signals and uplink A/N signals to besimultaneously adjusted, and implementation thereof will require acomplicated system and algorithm. Moreover, when one of the arrangementof the downlink control signal and the uplink A/N resource isdetermined, the other one is automatically determined, and it istherefore difficult to perform scheduling so that the arrangement ofboth becomes appropriate from the standpoint of the allocation blockprobability or resource utilization efficiency.

Another method is a method of using RRC (radio resource control)information or the like to allocate A/N resources to each terminalbeforehand.

As the A/N resource determination method when more A/N bits need to befed back at the time of carrier aggregation or the like, Rel.10 hasadopted a method of setting four A/N resource candidates by RRC anddynamically selecting A/N resources in subframe units using a 2-bit ARI(ACK/NACK resource indicator) included in the PDCCH (Non-PatentLiterature 3). FIG. 7 is a table that associates A/N resourcescandidates set according to RRC control information with ARI values. Theterminal determines A/N resources based on a value indicated by the ARIof the decoded PDCCH.

Adopting an ARI also to an E-PDCCH and selecting A/N resources in thesame way as that described above allows an A/N resource settingindependent of E-PDCCH scheduling. In this case, the same A/N resourcecandidate may be set for a plurality of E-PDCCH terminals and A/Nresources may be controlled by an ARI of E-PDCCH transmitted to eachterminal. Since there are a plurality of alternatives, it is possible toavoid A/N resources of the PDCCH terminal and A/N resources of aterminal for which a different E-PDCCH is set, and an allocation block.Moreover, since the allocation block can be avoided by adjusting theARI, it is unnecessary to readjust scheduling of PDCCH and E-PDCCH.

However, in the selection of A/N resources using the ARI, it is possibleto set only A/N resource candidates corresponding in number to bits ofthe ARI. In the case where the ARI has, for example, 2 bits, the numberof selectable A/N resources is 4. In consideration of a possibility thatcollision in A/N resources may occur with PDCCH terminals or terminalsfor which other E-PDCCHs are set, there is also a possibility that someof the four A/N resources may not be able to be used. For this reason,there is a problem that the ARI alone provides too small a number ofalternatives to perform flexible A/N resource control.

The number of A/N resource candidates can be increased by increasing thenumber of bits of the ARI. However, an excessive increase in the numberof ARI bits may lead to an increase of overhead of the E-PDCCH, and istherefore not desirable from the viewpoint of performance and coverage.

Thus, an object of the communication system according to Embodiment 1 isto simultaneously achieve two objectives: (1) preventing the number ofbits of the ARI from increasing and (2) increasing the number of A/Nresource candidates.

[Overview of Communication System]

The communication system of Embodiment 1, as shown in the example ofFIG. 5, includes one base station 100 and a plurality of terminals 200or the like within a cell.

[Configuration of Base Station 100]

FIG. 8 is a block diagram showing the main parts of base station 100.

Base station 100, as shown in FIG. 8, has control section 110 thatgenerates a plurality of pieces of control information to be transmittedto a plurality of terminals 200, respectively, and transmitting section120 that converts control information and transmission data to a radiotransmission signal and transmits the signal by radio via antenna 11.

Control section 110 generates control information for each terminal 200from downlink resource allocation information or the like. Controlsection 110 schedules control information to be transmitted to eachterminal 200 in the PDCCH or E-PDCCH. At this time, the E-PDCCH istransmitted in one configuration out of one or a plurality ofconfigurations set beforehand for terminal 200. The E-PDCCH terminal isnotified of which of notification A/N resource candidates previouslyspecified through RRC notification is used to transmit A/N using the ARIincluded in the E-PDCCH. Therefore, control section 110 generatescontrol information of the E-PDCCH terminal including the ARI andoutputs the control information to transmitting section 120.

Transmitting section 120 transmits, by radio, the signals through thechannels, which include transmission data and control data. That is,transmitting section 120 transmits, respectively, transmission data bythe PDSCH, PDCCH terminal control information by the PDCCH, and E-PDCCHterminal control information by the E-PDCCH.

FIG. 9 is a block diagram showing the details of the base station 100.

In detail, base station 100, as shown in FIG. 9, includes antenna 11,control information generation section 12, control information codingsection 13, modulation sections 14 and 17, data coding section 15,retransmission control section 16, subframe configuration section 18,IFFT section 19, CP appending section 20, radio transmitting section 21,and the like. Base station 100 also includes radio receiving section 22,CP removal section 23, despreading section 24, correlation processingsection 25, judgment section 26, and the like.

Of these constituent elements, control information generation section 12functions mainly as control section 110, and the constituent elementsfrom control information coding section 13 to radio transmitting section21 and data coding section 15 to radio transmitting section 21 functionmainly as transmitting section 120.

Base station 100 transmits the PDCCH, the E-PDCCH, and the PDSCH on thedownlink. Base station 100 also receives the PUCCH carrying the A/Nsignal on the uplink. In this case, to avoid having the descriptionbecome complex, the constituent elements related to the downlinktransmission of the PDCCH, E-PDCCH, and PDSCH, which are closelyconnected with the features of the present embodiment, and the uplinkreception of the PUCCH with respect to that downlink data are mainlyshown. The constituent elements related to uplink data reception areomitted in the illustration and descriptions.

The downlink control signal and data signal generated by base station100 are each separately encoded, modulated, and inputted to subframeconfiguration section 18.

First, the generation of the control signal will be described. Controlinformation generation section 12 generates the control information foreach terminal 200, from the resource allocation results (resourceallocation information) and the coding rate information of each terminal200 for which downlink allocation is to be done. The control informationfor each terminal 200 includes terminal ID information indicating forwhich terminal 200 the control information is intended. For example, theCRC bit masked by the ID number of terminal 200 that is the controlinformation notification destination is included in control informationas the terminal ID information. In this case, different information isincluded in the control information mapped onto the PDCCH and thecontrol information mapped onto the E-PDCCH. In particular, ARIindicating which A/N resource candidate notified beforehand using RRC isto be used is included in the control information mapped onto theE-PDCCH. The generated control information for each terminal 200 isinputted to control information coding section 13.

Control information coding section 13 independently encodes the controlinformation for each terminal 200 based on the coding rate information.The encoding may be done with the control information mapped onto thePDCCH and the control information mapped onto the E-PDCCH being eithersame or different. The output of control information coding section 13is inputted to modulation section 14.

Modulation section 14 independently modulates the control information ofeach terminal 200. The modulation may be done with the controlinformation mapped onto the PDCCH and the control information mappedonto the E-PDCCH being either same or different. The output ofmodulation section 14 is inputted to subframe configuration section 18.

Next, the generation of the data signal will be described. Data codingsection 15 appends a CRC bit that is masked based on the ID of eachterminal 200 to the data bit stream transmitted to each terminal 200 andperforms error correction coding. The output of data coding section 15is inputted to retransmission control section 16.

Retransmission control section 16 holds the coded transmission data foreach terminal 200 and outputs the transmission data to modulationsection 17 at the time of the first transmission. With respect toterminal 200 to which the NACK signal has been inputted from judgmentsection 26, that is, terminal 200 that will perform retransmission,retransmission control section 16 outputs the transmitted data forretransmission to modulation section 17.

Modulation section 17 performs data modulation of each of the data codedsequences for each terminal 200. The modulated streams are inputted tosubframe configuration section 18.

Subframe configuration section 18 maps the inputted control informationsequences and data sequences onto resources divided by the time andfrequency of a subframe based on the resource allocation information. Bydoing this, subframe configuration section 18 configures and outputssubframes to IFFT section 19. IFFT section 19 performs an IFFT (inversefast Fourier transform) on the transmission subframes that are inputtedthereto, thereby obtaining a time waveform, which is inputted to CPappending section 20.

CP appending section 20 appends a CP to each OFDM symbol within thesubframe and outputs the result to radio transmitting section 21.

Radio transmitting section 21 performs radio modulation of the inputtedsymbols to the carrier frequency band and transmits the modulateddownlink signal via antenna 11.

Radio receiving section 22 receives, as input, an A/N signal of terminal200 from antenna 11 and performs radio demodulation on the input signal.The demodulated downlink signal is inputted to CP removal section 23.

CP removal section 23 removes the CP from each SC-FDMA (singlecarrier-frequency-division multiple access) symbol within the downlinksignal. After removal of the CPs, the symbols are inputted todespreading section 24.

In order to extract the A/N of the target terminal 200 from the A/Nsignals of a plurality of terminals 200 that have been code multiplexed,despreading section 24 performs despreading by a correspondingorthogonal code. The despread signal is outputted to correlationprocessing section 25.

Correlation processing section 25 performs correlation processing by aZAC sequence in order to extract the A/N. The signal after correlationprocessing is inputted to judgment section 26.

Judgment section 26 judges whether the A/N of terminal 200 is ACK orNACK. If the judgment result indicates ACK, judgment section 26 promptsretransmission control section 16 to transmit the next data. If,however, the judgment result indicates NACK, judgment section 26 promptsretransmission control section 16 to perform retransmission.

[Configuration of Terminal 200]

FIG. 10 is a block diagram showing the main parts of a terminal.

Terminal 200 includes receiving section 230 that receives controlinformation and downlink data via antenna 41, control section 220 thatdetermines the resource used for transmitting the A/N signal, based onthe control information, and transmitting section 210 that transmits theA/N signal using the determined resource.

If terminal 200 is set to receive E-PDCCH control information, terminal200 becomes an E-PDCCH terminal, and if terminal 200 is set to receivePDCCH control information, terminal 200 becomes a PDCCH terminal.Terminal 200 may also be set to receive both. That is, terminal 200 setto receive both attempts to receive control information from bothE-PDCCH and PDCCH, and if terminal 200 can extract control informationof terminal 200 itself from the E-PDCCH, terminal 200 becomes an E-PDCCHterminal and if terminal 200 can extract control information of terminal200 itself from the PDCCH, terminal 200 becomes a PDCCH terminal. Ifterminal 200 receives no particular notification or specification,terminal 200 becomes a PDCCH terminal.

Furthermore, terminal 200 is notified of the configuration of theE-PDCCH in which control information of terminal 200 itself may bepossibly included from a higher layer such as RRC. The number of suchconfigurations may be one or plural. When E-PDCCHs of a plurality ofconfigurations are set, terminal 200 examines which of the respectiveconfigurations is used to transmit the E-PDCCH of terminal 200 itself.Base station 100 transmits the E-PDCCH to terminal 200 using one of theconfigurations.

Receiving section 230 receives received data via the PDSCH, and controlinformation via the E-PDCCH or PDCCH. That is, in the case of E-PDCCHterminal 200, receiving section 230 receives control informationincluding an ARI via the E-PDCCH, and in the case of PDCCH terminal,receives control information via the PDCCH. Receiving section 230outputs the received control information to control section 220.

In the case of E-PDCCH terminal 200, control section 220 identifieswhich of the A/N resources (RRC notification A/N resources) notifiedusing RCC or the like is used as an A/N signal transmission resource ofreceived data based on the two: the configuration of the receivedE-PDCCH and the value of the ARI. In the case of PDCCH terminal 200,control section 220 determines the A/N signal transmission resource inthe same manner as a conventional PDCCH terminal. Control section 220outputs the details of the determination to transmitting section 210.

Transmitting section 210 uses the determined resource to transmit theA/N signal of the received data by radio.

FIG. 11 is a block diagram showing the details of a terminal.

As shown in FIG. 11, specifically, terminal 200 includes antenna 41,radio receiving section 42, CP removal section 43, FFT section 44,extraction section 45, data demodulation section 46, data decodingsection 47, judgment section 48, control information demodulationsection 49, control information decoding section 50, control informationjudgment section 51, control processing section 52, A/N signalmodulation section 53, primary spreading section 54, IFFT section 55, CPappending section 56, secondary spreading section 57, multiplexingsection 58, and radio transmitting section 59. Terminal 200 alsoincludes IFFT section 60, CP appending section 61, and spreading section62.

Of the above constituent elements, control processing section 52 mainlyfunctions as control section 220. Constituent elements from A/N signalmodulation section 53 to radio transmitting section 59 mainly functionas transmitting section 210, and constituent elements from radioreceiving section 42 to judgment section 48 and from radio receivingsection 42 to control information judgment section 51 mainly function asreceiving section 230.

Terminal 200 receives, on the downlink, control information mapped ontothe PDCCH or E-PDCCH, and downlink data mapped onto the PDSCH. Terminal200 transmits the PUCCH on the uplink. In this case, to avoid having thedescription become complex, only the constituent elements related todownlink reception (specifically, PDCCH, E-PDCCH, and PDSCH), which areclosely connected with the features of the present embodiment, andrelated to the uplink transmission (specifically, PUCCH) with respect tothe downlink received data are indicated.

Radio receiving section 42 inputs the input from antenna 41 that hasreceived the downlink signal transmitted from the base station, performsradio demodulation, and outputs the demodulated signal to CP removalsection 43.

CP removal section 43 removes the CP from each OFDM symbol time waveformwithin the subframe and outputs the result to FFT section 44.

FFT section 44 performs an FFT (fast Fourier transform) on the receivedtime waveform in order to perform OFDM (orthogonal frequency divisionmultiplexing) demodulation, thereby obtaining a subframe in thefrequency domain. The obtained received subframe is inputted toextraction section 45.

Extraction section 45 extracts the control information intended for theterminal itself from either the PDCCH region or the E-PDCCH region.Information indicating in which one of the PDCCH and the E-PDCCH thecontrol information is included is specified beforehand from basestation 100 (not shown). Extraction section 45 extracts one or aplurality of control information candidates from a control informationregion onto which the control information of the terminal itself ispossibly mapped using the coding rate information of the controlinformation, and outputs the candidate to control informationdemodulation section 49. When a result is obtained from controlinformation judgment section 51, extraction section 45 extracts a datasignal intended for the terminal from the received subframe based on theresource allocation result included in the control information intendedfor the terminal. The obtained data signal is inputted to datademodulation section 46.

Control information demodulation section 49 demodulates one or morepieces of inputted control information and outputs the result to controlinformation decoding section 50.

Control information decoding section 50 decodes one or more inputteddemodulated sequences using the coding rate information of the controlinformation. The decoding result is inputted to control informationjudgment section 51.

Control information judgment section 51 judges, from the one or moredecoding results, the control information intended for the terminal,using the terminal ID information. The judgment uses, for example, theCRC bit that is masked by the ID information of the terminal itselfincluded in the control information. If there is control informationintended for the terminal itself, control information judgment section51 outputs that control information to extraction section 45. Controlinformation judgment section 51 outputs the control information tocontrol processing section 52.

Control processing section 52 operates differently between the case ofPDCCH terminal 200 and the case of E-PDCCH terminal 200.

In the case of PDCCH terminal 200, control processing section 52 obtainsthe resource number of the A/N signal based on equation 1 from thenumber of the resource (CCE) onto which the control information ismapped. From the obtained A/N signal resource number, control processingsection 52 determines the spreading codes used for primary spreading,secondary spreading, and the reference signal, and the frequencyresource block (RB) transmitting the PUCCH. This information is inputtedto primary spreading section 54, secondary spreading section 57, and tospreading section 62 of the reference signal.

On the other hand, in the case of E-PDCCH terminal 200, controlprocessing section 52 determines which of the A/N resource candidatesnotified as RRC control information is used based on the two: theconfiguration of the received E-PDCCH and the value indicated by the ARIincluded in the control information. In this case, it is assumed thatthe RRC notification A/N resource is specified to terminal 200 by basestation 100 beforehand (not shown). Control processing section 52determines each spreading code used for primary and secondary spreadingand the reference signal corresponding to the specified A/N resourcenumber, and the frequency resource block (RB) for transmitting thePUCCH. Control processing section 52 outputs the corresponding spreadingcode to primary spreading section 54, secondary spreading section 57,and the reference signal spreading section 62.

Data demodulation section 46 demodulates the inputted data signalintended for the terminal itself. The result of the demodulation isinputted to data decoding section 47.

Data decoding section 47 decodes the inputted demodulated data. Theresult of the decoding is inputted to judgment section 48.

Judgment section 48 uses the CRC masked by the ID of terminal 200 tojudge whether or not the decoding result is correct. If the decodingresult is correct, judgment section 48 outputs the ACK signal to A/Nsignal modulation section 53 and extracts the received data. If thedecoding result is not correct, judgment section 48 outputs the NACKsignal to A/N signal modulation section 53.

A/N signal modulation section 53, depending upon whether the inputsignal is ACK or NACK, generates modulated symbols having differentvalues. The generated demodulated symbol is inputted to primaryspreading section 54.

Primary spreading section 54 uses the ZAC sequence inputted from controlprocessing section 52 to perform primary spreading of the A/N signal andoutputs the A/N signal after primary spreading to IFFT section 55. Inthis case, because the cyclic shift value used for cyclic shift hoppingdiffers in units of SC-FDMA, primary spreading section 54 uses adifferent cyclic shift value for each SC-FDMA symbol to perform primaryspreading of the A/N signal.

IFFT section 55 performs an IFFT of each SC-FDMA symbol inputted fromprimary spreading section 54 and outputs the obtained time waveform toCP appending section 56.

CP appending section 56 appends a CP to each inputted SC-FDMA timewaveform and outputs this signal to secondary spreading section 57.

Secondary spreading section 57 uses a block-wise spreading code sequenceto perform secondary spreading of the SC-FDMA time waveform after theappending the CP. The spreading code used is a code specified by controlprocessing section 52. The sequence that has been subjected to secondaryspreading is inputted to multiplexing section 58.

Multiplexing section 58 time-multiplexes the two inputted sequencesreceived as input from spreading section 62 for the reference signal andsecondary spreading section 57, thereby generating a PUCCH subframe. Thetime multiplexed signal is inputted to radio transmitting section 59.

Radio transmitting section 59 performs radio modulation of the inputtedsignal to the carrier frequency band and transmits the uplink signal byradio from antenna 41.

IFFT section 60 performs IFFT on the reference signal and outputs thetime waveform obtained to CP appending section 61.

CP appending section 61 appends a CP to the time waveform of theinputted reference signal and outputs this signal to spreading section62.

Spreading section 62 spreads the time waveform with the CP. Thespreading code used is a code specified by control processing section52. The spread sequence is inputted to multiplexing section 58.

[Operation]

The processing flow of base station 100 and terminal 200 in Embodiment 1will be described by step (1) through step (6).

FIG. 12 is a table showing the ARI included in the E-PDCCH and A/Nresources determined by the E-PDCCH configuration.

Step (1): Before transmission or reception of a PDSCH, base station 100notifies terminal 200 that can transmit control information using anE-PDCCH of the use of the E-PDCCH. Notification is not particularlynecessary for terminal 200 that does not perform transmission using theE-PDCCH. When terminal 200 receives no notification or cannot recognizenotification in particular, terminal 200 receives control informationassuming that the control information is transmitted using the PDCCH. Inaddition, base station 100 notifies terminal 200 that may possiblytransmit control information using the E-PDCCH of the configuration ofthe E-PDCCH which may be possibly used before transmission or receptionof the PDSCH. For example, in FIG. 12, all of the three configurationsare set for certain terminal 200, configuration A and configuration Bare set for certain terminal 200, and only configuration A is set forcertain terminal 200. In addition, base station 100 notifies A/Nresource candidates determined by the ARI value and the E-PDCCHconfiguration before transmission or reception of the PDSCH. These A/Nresource candidates are A to D, W to Z, and O to R in FIG. 12. An RRCcontrol signal or the like is used for this notification.

Step (2): Base station 100 determines terminal 200 for assignment ofdata in each subframe and performs scheduling within the PDSCH. Thescheduling uses, in addition to the amount of traffic to each terminal200, the CSI feedback and sounding reference signal (SRS) transmitted byterminal 200 and the like.

Step (3): Base station 100 generates control information including thescheduling results intended for each terminal 200 and schedules thecontrol information in the PDCCH or the E-PDCCH. Base station 100determines a configuration in which the E-PDCCH is transmitted forterminals 200 for which a plurality of E-PDCCH configurations are setand performs scheduling based on the configuration.

Base station 100 confirms whether or not collision occurs in A/Nresources between all terminals 200 for which control information isscheduled. When collision in A/N resources occurs, base station 100examines whether or not collision in A/N resources can be prevented bychanging the scheduling result of the PDCCH, the ARI value of theE-PDCCH and the E-PDCCH configuration or the like. In the case wherecollision in A/N resources cannot be avoided, base station 100 gives upscheduling for terminal 200 in which collision occurs (allocationblock).

Step (4): When the control information scheduling for all terminals 200is completed, base station 100 transmits, by radio, PDCCH and E-PDCCHcontrol information and PDSCH downlink data using the downlink.

Step (5): Terminal 200 obtains from the received signal the controlinformation intended for terminal 200 and extracts and decodes the datasignal. Terminal 200 for which control information may have beentransmitted using the E-PDCCH in particular also confirms in whichconfiguration out of one or a plurality of usable configurations thecontrol information has been transmitted. Furthermore, terminal 200identifies the code and frequency resources for transmitting the A/Nsignal corresponding to the received data signal based on the controlinformation. E-PDCCH terminal 200 in particular determines which of theA/N resource candidate notified beforehand by RRC is used based on theconfiguration of the E-PDCCH intended for the terminal and the value ofthe ARI included in the E-PDCCH.

Step (6): Terminal 200 identifies either ACK or NACK, in accordance withthe judgment result of the data signal, and transmits the A/N signalusing the A/N resources (code and frequency resources) identified asnoted above.

[Effects]

As described above, according to base station 100 and terminal 200 ofEmbodiment 1, it is possible to increase the number of A/N resourcecandidates without increasing the number of ARI bits for terminal 200for which a plurality of E-PDCCH configurations are set.

In addition, according to Embodiment 1, it is possible to graduallyincrease the number of A/N resource candidates as required by adding aconfiguration usable for the E-PDCCH terminal in accordance with thecommunication environment or terminal situation or the like.

In addition, according to Embodiment 1, A/N resource candidates, thatis, A to D, W to Z, and O to R in FIG. 12 are all A/N resources notifiedbeforehand using RRC control information or the like. Therefore,compared to the allocation method as shown in equation 2 whereby A/Nresources are determined by resources for which E-PDCCH is scheduledsuch as eCCE number, base station 100 can easily adjust A/N resources.This allows the circuit scale of base station 100 to be reduced.

Modification Example 1

The communication system of Embodiment 1 can obtain similar effects alsowhen the following modification is made thereto.

The number of ARI bits may be changed by the E-PDCCH configuration.

FIG. 13A and FIG. 13B show an example where the number of ARI bitsdiffers depending on the configuration. FIG. 13A shows an example whereonly configuration A has a 2-bit ARI and other configurations have a1-bit ARI. FIG. 13B shows an example where only configuration A has a1-bit ARI and other configurations have a 2-bit ARI.

By so doing, it is also possible to obtain an effect of reducingoverhead by reducing the number of ARI bits in addition to effectssimilar to those of Embodiment 1. For example, in an operation in whichconfiguration A is frequently used for E-PDCCH, and configurations B andC are used only for fewer E-PDCCH terminals, fewer terminals 200transmit E-PDCCH using configurations B and C, and it is therebypossible to increase the number of ARI bits of configuration A andreduce the number of ARI bits of configurations B and C. At this time,the overhead corresponding to ARI bits can be reduced, but since thereare fewer E-PDCCH terminals of configurations B and C, it is possible toreduce deterioration of the allocation block rate obtained by reducingthe number of ARI bits. On the other hand, while using the sameoperation, in an environment in which high priority is given to areduction of overhead of the ARI bits rather than deterioration of theallocation block rate, the number of the ARI bits of configuration A maybe conversely reduced and the number of the ARI bits of configurations Band C may be increased. It is thereby possible to achieve effectssimilar to those of Embodiment 1 and at the same time reduce the numberof ARI bits of configuration A, reduce the number of information bitsincluded in control information, and thereby improve receiving qualityof E-PDCCH configuration A in which transmission or reception in variousenvironments is assumed.

Modification Example 2

The communication system of Embodiment 1 can achieve the same effecteven if the following changes are made thereto.

A range of A/N resource candidates specifiable by an ARI may be limitedfor each E-PDCCH configuration.

FIGS. 14A and 14B show an example where a range in which A/N resourcecandidates can be set by an E-PDCCH configuration is limited. In theexample of FIG. 14A, the range of A/N resources in configurations B andC is limited to only a region different from the range of A/N resourcesof a PDCCH terminal and in the example of FIG. 14B, the range of A/Nresources in configuration A is limited to the same region as the rangeof A/N resources of a PDCCH terminal.

By so doing, it is possible to obtain an effect of reducing overhead ofan RRC control signal caused by limiting the A/N resource candidatesettable range in addition to effects similar to those of Embodiment 1.For example, in an operation in which configuration A is a configurationfrequently used for E-PDCCH, and configurations B and C areconfigurations only used for fewer E-PDCCH terminals, fewer terminals200 transmit E-PDCCH using configuration B or C. Thus, by widening theA/N resource candidate settable range of configuration A and narrowingthe A/N resource candidate settable range of configurations B and C, itis possible to reduce overhead of RRC while obtaining effects similar tothose of Embodiment 1. On the other hand, while using the sameoperation, the A/N resource candidate settable range of configuration Amay be narrowed and the A/N resource candidate settable range ofconfigurations B and C may be widened. In this case, since configurationA is used which has a narrow setting range as long as no allocationblock occurs, it is possible to secure resources allocatable to thePUSCH and improve uplink throughput.

Embodiment 2

[Overview of Communication System]

In Embodiment 2, E-PDCCHs are set as a PRB set composed of one or aplurality of PRBs for a terminal. In the terminal, E-PDCCHs aretransmitted or received in the set PRB set.

Furthermore, one or a plurality of PRB sets of E-PDCCHs is/are set foreach E-PDCCH terminal. Information on the set PRB set is notified from abase station to a terminal using RRC control information or the like.The number of the set PRB sets can be changed for each terminal.

FIGS. 15A and 15B illustrate examples where two PRB sets are set withina subframe. FIG. 15A illustrates an example where the two PRB sets havethe same PRB frequency interval and FIG. 15B illustrate an example wherethe two PRB sets have different PRB frequency intervals. It is assumedin Embodiment 2 that such a PRB frequency interval is settable for eachPRB set or every plurality of PRB sets. Note that a plurality ofpredetermined PRB sets may be defined and PRB sets to be used may beselected from among those PRB sets.

In the following, to avoid having the description become complex,constituent elements that are the same as in Embodiment 1 are assignedthe same reference signs, and only the difference with respect toEmbodiment 1 will be described.

[Configuration of Base Station]

The configuration of base station 100 differs from Embodiment 1 mainlyin the difference in the processing done by control section 110, withother parts being the same as Embodiment 1. The details of theprocessing done by control section 110 will be described in detail inthe description of operation to follow.

[Configuration of Terminal]

The configuration of terminal 200 differs mainly by the difference inthe processing done by control section 220, with other parts being thesame as in Embodiment 1. The details of the processing done by controlsection 220 will be described in detail in the description of operationto follow.

[Operation]

The processing flow of base station 100 and terminal 200 in Embodiment 2will be described by step (1) through step (6).

FIG. 16 is a table showing the ARI included in the E-PDCCH and A/Nresources determined by the PRB set.

Step (1): Before transmission or reception of the PDSCH, base station100 notifies terminal 200 that can transmit control information usingthe E-PDCCH of the setting of the PRB set. Note that in the case of thePRB set that can be used for all E-PDCCH terminals, the setting thereofneed not be notified. The setting of the PRB set to be notified and thenumber of PRB sets are determined for each terminal 200. For example, inFIG. 15, PRB sets A and B are set for certain terminal 200 and only PRBset A is set for certain terminal 200. Before transmission or receptionof a PDSCH, base station 100 notifies A/N resource candidates determinedby the ARI value and the PRB set of the E-PDCCH. The A/N resourcecandidates are A to D and W to Z in FIG. 16. An RRC control signal orthe like is used for these notifications.

Step (2): Base station 100 determines terminal 200 to which data isassigned in each subframe and schedules it in the PDSCH. A CSI feedbackor sounding reference signal (SRS) or the like transmitted by terminal200 is used for scheduling in addition to the amount of traffic to eachterminal 200.

Step (3): Base station 100 generates control information including thescheduling results intended for each terminal 200 and schedules it ontothe PDCCH or the E-PDCCH. Base station 100 determines a PRB set fortransmitting the E-PDCCH for terminal 200 for which a plurality ofE-PDCCH PRB sets are set and performs scheduling in the PRB sets.

Base station 100 confirms whether or not collision in A/N resourcesoccurs between all scheduled terminals 200. When collision in A/Nresources occurs, base station 100 confirms whether or not it ispossible to avoid collision in A/N resources by changing the schedulingresults of the PDCCH, the ARI value of the E-PDCCH and PRB set of theE-PDCCH or the like. When collision in A/N resources cannot be avoided,base station 100 gives up scheduling for terminal 200 in which collisionoccurs (allocation block).

Step (4): When the mapping of control information in all terminals 200is completed, base station 100 transmits, by radio, control informationof the PDCCH and E-PDCCH and downlink data of the PDSCH on the downlink.

Step (5): Terminal 200 obtains from the received signal the controlinformation intended for terminal 200 and extracts and decodes the datasignal. Terminal 200 to which the control information might have beenpossibly transmitted using the E-PDCCH in particular also confirms bywhich PRB set out of one or a plurality of usable PRB sets the controlinformation was transmitted. Terminal 200 identifies code and frequencyresources for transmitting an A/N signal corresponding to the receiveddata signal based on the control information. E-PDCCH terminal 200 inparticular determines which of A/N resource candidates notifiedbeforehand using RRC is used based on the PRB set whereby the E-PDCCHintended for the terminal itself is transmitted and the value of the ARIincluded in the E-PDCCH (e.g., see FIG. 17).

Step (6): Terminal 200 identifies either ACK or NACK, in accordance withthe judgment result of the data signal, and transmits the A/N signalusing the A/N resources (code and frequency resources) identified asnoted above.

[Effects]

As described above, base station 100 and terminal 200 according toEmbodiment 2 can increase the number of A/N resource candidates forterminal 200 for which a plurality of E-PDCCH PRB sets are set withoutincreasing the number of ARI bits. According to Embodiment 2, since thenumber of A/N resource candidates selectable by terminal 200 for which aplurality of PRB sets are set is increased, it is also possible toreduce the probability that A/N resources of terminal 200 for which onlya single PRB set is set will become an allocation block.

Conversely, when the number of A/N resources is greater than the numberof terminals 200 to which the A/N resources are allocated such as whenthe number of terminals 200 is small or when the number of terminals 200allocated to the downlink in the same subframe is small, it is possibleto reduce the number of PRB sets used by limiting the A/N resources tobe used to A to D, for example. This makes it possible to increase thenumber of downlink PRBs for transmitting data and thereby increase thethroughput per terminal.

According to Embodiment 2, by adding a PRB set usable for the E-PDCCHterminal in accordance with the communication environment or terminalsituation or the like, it is possible to gradually increase the numberof A/N resource candidates as required.

According to Embodiment 1, A/N resource candidates, that is, A to D andW to Z in FIG. 16 are all A/N resources notified beforehand by RRCcontrol information or the like. Therefore, base station 100 can easilyadjust A/N resources compared to the allocation method as shown inequation 2 whereby A/N resources are determined by resources such aseCCE numbers for which the E-PDCCH is scheduled. Moreover, this allowsthe circuit scale of base station 100 to be reduced.

Modification Example 1

The communication system according to Embodiment 2 can achieve the sameeffect even if the following change is made thereto.

The number of ARI bits may be changed according to the PRB set of theE-PDCCH. For example, the number of bits of the ARI included in theE-PDCCH transmitted in PRB set A is 2 and the number of bits of the ARIincluded in the E-PDCCH transmitted in PRB set B is 1, or the like.Alternatively, the number of bits of the ARI may be set to 0 dependingon the PRB set. At this time, one A/N resource notified as RRC controlinformation is used.

By so doing, it is possible to obtain an effect of reducing overhead byreducing the number of ARI bits in addition to effects similar to thoseof Embodiment 2. For example, in an operation in which PRB set A isfrequently used for E-PDCCH, and PRB set B is only used for fewerE-PDCCH terminals, fewer terminals 200 transmit E-PDCCH using PRB set B,and it is thereby possible to increase the number of ARI bits of PRB setA and reduce the number of ARI bits of PRB set B. At this time, theoverhead corresponding to the ARI bits can be reduced, but since thereare fewer E-PDCCH terminals of PRB set B, it is possible to reducedeterioration of the allocation block rate resulting from reducing thenumber of ARI bits. On the other hand, while using the same operation,in an environment in which high priority is given to a reduction ofoverhead of the ARI bits rather than deterioration of the allocationblock rate, the number of the ARI bits of PRB set A may be converselyreduced and the number of the ARI bits of PRB set B may be increased. Itis thereby possible to achieve effects similar to those of Embodiment 1and at the same time reduce the number of ARI bits of PRB set A, reducethe number of information bits included in control information, andthereby improve receiving quality of E-PDCCH PRB set A.

Modification Example 2

The communication system of Embodiment 2 can achieve the same effecteven if the following change is made thereto.

The range of A/N resource candidates specifiable by an ARI may belimited for each PRB set of the E-PDCCH.

By so doing, it is possible to obtain an effect of reducing overhead ofRRC control signals by limiting the A/N resource candidate setting rangein addition to effects similar to those of Embodiment 2. For example, inan operation in which PRB set A is a PRB set frequently used forE-PDCCH, and PRB set B is a PRB set only used for fewer E-PDCCHterminals, fewer terminals 200 transmit E-PDCCH using PRB set B. Thus,by widening the A/N resource candidate settable range of PRB set A andnarrowing the A/N resource candidate settable range of PRB set B, it ispossible to reduce the overhead of RRC while obtaining effects similarto those of Embodiment 2. On the other hand, while using the sameoperation, the range in which A/N resource candidates of PRB set A canbe set may be narrowed and the range in which A/N resource candidates ofPRB set B or C can be set may be widened. In this case, since PRB set Ahaving a narrow setting range is used unless an allocation block occurs,it is possible to secure resources allocatable to the PUSCH and improvethe uplink throughput.

[Variation]

In Embodiment 2, a certain PRB may be included in two or more differentPRB sets. FIG. 18 illustrates an example. Thus, when the E-PDCCH istransmitted using a PRB included in both PRB sets, terminal 200 cannotdetermine which of the two A/N resources determined by the ARI and twoPRB sets should be used. Since base station 100 cannot know which A/Nresource is used by terminal 200 for transmission, base station 100 hasto reserve both of the two A/N resources for terminal 200. This leads todeterioration of A/N resource utilization efficiency.

Thus, when the E-PDCCH is transmitted or received using PRB included inall PRB sets, Embodiment 2 can solve the above-described problem thatterminal 200 cannot determine which of the two A/N resources should beused, by defining that the E-PDCCH is always regarded as having beentransmitted from PRB set A. This allows terminal 200 to always determineone A/N resource to be specified by the ARI irrespective of the settingof the PRB set and it is thereby possible to prevent PUCCH resourceutilization efficiency from deteriorating.

Note that in Embodiment 2, when the E-PDCCH is transmitted or receivedusing PRBs included in all PRB sets, A/N resources corresponding to aPRB set having a smaller frequency interval of PRBs included in the PRBset may be used. The greater the spread of the frequency interval of thePRB set, the higher is the frequency diversity effect, and thereforeE-PDCCH terminals of a variety of communication environments andcommunication qualities can receive the E-PDCCH. Therefore, an operationusing mainly a PRB set having a large frequency interval may beconsidered. In such a case, a PRB set having a greater frequencyinterval is more likely to accommodate more terminals 200. Therefore,when the E-PDCCH is transmitted or received using PRBs included in allPRB sets, by using A/N resources corresponding to the PRB set having thesmaller frequency interval of PRBs included in the PRB set, it ispossible to reduce the probability of collision of A/N resources.Moreover, by so doing, A/N resources corresponding to the PRB set havinga greater frequency interval of PRBs included in the PRB set are usable,and more terminals 200 can be accommodated.

Alternatively, in Embodiment 2, when the E-PDCCH is transmitted orreceived using PRBs included in all PRB sets, A/N resourcescorresponding to the PRB set having the greater frequency interval ofPRBs included in the PRB set may be used. In an operation in whichE-PDCCH terminal 200 within a cell has a relatively good communicationenvironment and communication quality and does not require a largefrequency diversity effect, using a PRB set having a smaller frequencyinterval allows a greater continuous band of downlink PDSCH to beobtained, and it is thereby possible to achieve high downlink throughputper terminal. Therefore, in this case, an operation of mainly using aPRB set having a smaller frequency interval may be considered. In such acase, a PRB set having a smaller frequency interval is more likely toaccommodate more terminals 200. Therefore, when the E-PDCCH istransmitted or received using PRBs included in all PRB sets, by usingA/N resources corresponding to the PRB set having the greater frequencyinterval of PRBs included in the PRB set, it is possible to reduce thepossibility of collision of A/N resources. Moreover, by so doing, it ispossible to use A/N resources corresponding to the PRB set having thesmaller frequency interval of PRBs included in the PRB set, and therebyaccommodate more terminals 200.

Embodiment 3

[Overview of Communication System]

In Embodiment 3, E-PDCCH is transmitted or received to/from a terminalby a search space (SS) composed of one or a plurality of PRBs. Theterminal receives the E-PDCCH in a set search space.

One or a plurality of search spaces is/are set for each E-PDCCHterminal. A search space common to many terminals 200 is called “commonsearch space (CSS)” and a search space common to only one or fewerterminals 200 is called “UE-specific search space (US S).” Informationon the set search space is notified from base station 100 to terminal200 through RRC control information or the like. The number of searchspaces to be set can be changed from one terminal to another.

FIG. 19 shows an example where two search spaces: CSS and USS, are setwithin a subframe. Since a CSS accommodates even terminal 200 having alow average received signal to interference noise power ratio (SINR) ofE-PDCCH or terminal 200 unable to perform E-PDCCH frequency schedulingwith high accuracy, the CSS is likely to be arranged with a large PRBinterval so as to obtain a frequency diversity effect. On the otherhand, a USS accommodates terminal 200 which need not be accommodated bythe CSS or a terminal from which a frequency scheduling effect can beobtained, the USS is likely to be arranged with a narrow PRB intervaland concentrated on a specific frequency band. A PRB for setting asearch space may be settable for each terminal 200 or may be setbeforehand.

To avoid having the description become complex, components similar tothose in Embodiment 2 will be assigned the same reference numerals andonly the difference with respect to Embodiment 2 will be describedbelow.

[Configuration of Base Station]

The configuration of base station 100 differs from Embodiment 1 mainlyin the difference in the processing done by control section 110, withother parts being the same as Embodiment 1. The details of theprocessing done by control section 110 will be described in detail inthe description of operation to follow.

[Configuration of Terminal]

The configuration of terminal 200 differs from Embodiment 1 mainly inthe difference in the processing done by control section 220, with otherparts being the same as Embodiment 1. The details of the processing doneby control section 220 will be described in detail in the description ofoperation to follow.

[Operation]

A processing flow of base station 100 and terminal 200 according topresent Embodiment 3 will be described in steps (1) to (6).

FIG. 20 is a table showing ARIs included in the E-PDCCH and A/Nresources determined by search spaces of the E-PDCCH when two searchspaces CSS and USS are set.

Step (1): Before transmission or reception of a PDSCH, base station 100notifies terminal 200 that can transmit control information by anE-PDCCH of the setting of a search space. Note that setting informationof a CSS usable for all E-PDCCH terminals may be assumed to be definedbeforehand. The setting of search spaces and the number of search spacesare determined individually for each terminal 200. For example, in FIG.19, a CSS and USS are set for certain terminal 200 and only a CSS is setfor certain terminal 200. Base station 100 notifies A/N resourcecandidates determined by an ARI value and search space beforetransmission or reception of PDSCH. These A/N resource candidates are Ato D and W to Z in FIG. 20. An RRC control signal or the like is usedfor these notifications.

Step (2): Base station 100 determines terminal 200 to which data isassigned in each subframe and schedules it in the PDSCH. In addition tothe amount of traffic to each terminal 200, CSI feedback or soundingreference signal (SRS) or the like transmitted by terminal 200 is alsoused for scheduling.

Step (3): Base station 100 generates control information includingscheduling results for each terminal 200 and schedules the controlinformation for the PDCCH or E-PDCCH. For terminal 200 for which aplurality of search spaces are set, base station 100 determines a searchspace for transmitting E-PDCCH and performs scheduling in the searchspace.

Base station 100 checks whether or not collision in A/N resources occursbetween all scheduled terminals 200. When collision in A/N resourcesoccurs, base station 100 changes scheduling results of the PDCCH, ARIvalue of the E-PDCCH and search space of the E-PDCCH or the like, andthereby confirms whether or not it is possible to avoid collision in A/Nresources. When collision in A/N resources cannot be avoided, basestation 100 gives up the scheduling for terminal 200 in which collisionoccurs (allocation block).

Step (4): When the mapping of control information by all terminals 200is completed, base station 100 transmits, by radio, control informationof the PDCCH and E-PDCCH, and downlink data of the PDSCH on the downlinkby radio.

Step (5): Terminal 200 obtains control information intended for terminal200 itself from the received signal, and extracts and decodes a datasignal. Terminal 200 to which the control information might have beenpossibly transmitted using the E-PDCCH in particular also confirms bywhich search space out of one or a plurality of search spaces setbeforehand and usable the control information was transmitted. Terminal200 identifies code and frequency resources for transmitting A/N signalscorresponding to the received data signal based on the controlinformation. E-PDCCH terminal 200 in particular determines which of A/Nresource candidates notified beforehand by RRC is used based on thesearch space whereby the E-PDCCH intended for the terminal itself istransmitted and the value of the ARI included in the E-PDCCH (e.g., seeFIG. 21).

Step (6): Terminal 200 identifies ACK or NACK depending on the judgmentresult of the data signal and transmits an A/N signal using theabove-described identified A/N resources (code and frequency resources).

[Effects]

As described above, base station 100 and terminal 200 according toEmbodiment 3 can increase the number of A/N resource candidates withoutincreasing the number of ARI bits for terminal 200 for which a pluralityof E-PDCCH search spaces are set. Moreover, according to Embodiment 3,since the number of A/N resource candidates selectable by terminal 200for which a plurality of search spaces are set is increased, it ispossible to reduce the probability that A/N resources of terminal 200for which only a single search space is set will become an allocationblock.

Conversely, when the number of A/N resources is greater than the numberof terminals 200 allocated such as when the number of terminals 200 issmall or when the number of terminals 200 allocated on the downlink inthe same subframe is small, it is possible to reduce the number ofsearch spaces used by limiting the A/N resources used to A to D. Thismakes it possible to increase the number of downlink PRBs fortransmitting data and thereby increase the throughput per terminal.

Furthermore, according to Embodiment 3, by additionally setting searchspaces usable for the E-PDCCH terminal according to the communicationenvironment or terminal situation or the like, it is possible togradually increase the number of A/N resource candidates as required.

According to Embodiment 3, A/N resource candidates, that is, A to D, andW to Z in FIG. 20 are all A/N resources notified beforehand using RRCcontrol information or the like. Therefore, base station 100 can easilyadjust A/N resources compared to the allocation method as shown inequation 2 whereby A/N resources are determined by resources such aseCCE numbers for which the E-PDCCH is scheduled. Moreover, this allowsthe circuit scale of base station 100 to be reduced.

Modification Example 1

The communication system of Embodiment 3 can achieve the same effecteven if the following changes are made thereto.

The number of ARI bits may be changed according to the search space ofthe E-PDCCH. For example, the number of bits of the ARI included in theE-PDCCH transmitted by CSS is 2 and the number of bits of the ARIincluded in the E-PDCCH transmitted by USS is 1 or the like.Alternatively, the number of bits of the ARI may be set to 0 dependingon the search space. At this time, one A/N resource is always used asRRC control information.

By so doing, it is possible to reduce overhead of a CSS and improvereceiving quality of the E-PDCCH transmitted by the CSS in addition toeffects similar to those of Embodiment 3. Moreover, it is possible toexpand the coverage of the CSS and allow terminals having variousaverage reception SINRs to receive the E-PDCCH. Conversely, in anoperation in which expansion of the coverage is unnecessary, it ispossible to increase the degree of freedom of terminal 200 thattransmits or receives the E-PDCCH by the CSS in selecting A/N resourcesby increasing the number of bits of the ARI included in the CSS andreduce the allocation block rate.

Modification Example 2

The communication system of Embodiment 3 can achieve the same effecteven if the following changes are made thereto.

The range of A/N resource candidates specifiable by the ARI may belimited for each search space in which an E-PDCCH is transmitted orreceived.

By so doing, it is possible to obtain an effect of reducing overhead ofan RRC control signal by limiting the A/N resource candidate settablerange in addition to effects similar to those of Embodiment 3. Forexample, in such an operation in which a CSS is a search spacefrequently used for the E-PDCCH and a USS is a search space used foronly fewer E-PDCCH terminals, fewer terminals 200 transmit the E-PDCCHusing the USS. Thus, by widening the A/N resource candidate settablerange of the CSS and narrowing the USS A/N resource candidate settablerange, it is possible to reduce overhead of RRC while obtaining effectssimilar to those of Embodiment 3. On the other hand, even in the sameoperation, the CSS A/N resource candidate settable range may be narrowedand the USS A/N resource candidate settable range may be widened. Inthis case, since the CSS having a narrow setting range is used unless anallocation block occurs, it is possible to secure resources allocatableto a PUSCH and improve the uplink throughput. In such an operation inwhich the USS is a search space frequently used for the E-PDCCH and theCSS is a search space used for only fewer E-PDCCH terminals, manyterminals 200 transmit E-PDCCHs using the USS. Thus, by widening the USSA/N resource candidate settable range and narrowing the CSS A/N resourcecandidate settable range, it is possible to reduce overhead of RRC whileobtaining effects similar to those of Embodiment 3.

[Variation]

In Embodiment 3, there is a possibility that a certain PRB may besimultaneously included in two or more search spaces. When an E-PDCCH istransmitted by PRBs included in all search spaces, terminal 200 cannotdetermine which of a plurality of A/N resources determined by the ARIand search space should be used. Since base station 100 does not knowwhich A/N resource is used by terminal 200 for transmission, basestation 100 needs to reserve both of the two A/N resources for terminal200. This deteriorates the A/N resource utilization efficiency.

Thus, when an E-PDCCH is transmitted or received by PRBs included in allsearch spaces, Embodiment 3 defines that the E-PDCCH should always betransmitted from CSS, and can thereby solve the problem that terminal200 cannot determine which A/N resource should be used. This allowsterminal 200 to always determine one A/N resource to be specified by theARI irrespective of the setting of search spaces, and can therebyprevent deterioration of PUCCH resource utilization efficiency.

Note that Embodiment 3 may also be configured to use A/N resourcescorresponding to a USS when an E-PDCCH is transmitted or received byPRBs included in all search spaces. Since a CSS is an E-PDCCH receivedover a wide range, the corresponding A/N resources may also possiblyhave been frequently used. Thus, by defining that A/N resourcescorresponding to a USS are used when an E-PDCCH is transmitted orreceived by PRBs included in all search spaces, it is possible toallocate A/N resources which are more likely to be available and therebyreduce the probability of collision in A/N resources. Furthermore, by sodoing, A/N resources of a CSS are made available and more terminal 200can be accommodated.

Embodiment 4

[Overview of Communication System]

In Embodiment 4, an E-PDCCH is transmitted or received to/from aterminal in a distributed mode or localized mode. The distributed modeis a mode in which the E-PDCCH is arranged and transmitted over two ormore PRBs, and the localized mode is a mode in which the E-PDCCH isarranged and transmitted on one PRB.

Both or one of the distributed mode and localized mode are/is set foreach E-PDCCH terminal. In the distributed mode, single controlinformation is arranged over a plurality of PRBs, and it is therebypossible to obtain a high frequency diversity effect. In the localizedmode, since control information is arranged on only a single PRB, itsfrequency diversity effect is small but a frequency scheduling effectand interference avoidance effect can be obtained. Information on whichtransmission mode can be used is notified from base station 100 toterminal 200 by RRC control information or the like.

FIG. 22 shows an example of a case where E-PDCCHs transmitted in thedistributed mode and localized mode are located within a subframe. Sincespread PRBs are used in the distributed mode, a frequency diversityeffect can be obtained. On the other hand, in the localized mode, sincecontrol information is transmitted on a single PRB, no frequencydiversity effect is obtained, but a frequency scheduling effect andinterference avoidance effect can be obtained.

In the following, to avoid having the description become complex,constituent elements that are the same as in Embodiment 1 are assignedthe same reference signs, and only the difference with respect toEmbodiment 3 will be described.

[Configuration of Base Station]

The configuration of base station 100 differs from Embodiment 1 mainlyin the difference in the processing done by control section 110, withother parts being the same as Embodiment 1. The details of theprocessing done by control section 110 will be described in detail inthe description of operation to follow.

[Configuration of Terminal]

The configuration of terminal 200 differs mainly by the difference inthe processing done by control section 220, with other parts being thesame as in Embodiment 1. The details of the processing done by controlsection 220 will be described in detail in the description of operationto follow.

[Operation]

The processing flow of base station 100 and terminal 200 in Embodiment 4will be described by step (1) through step (6).

FIG. 23 is a table showing A/N resources determined by an ARI includedin an E-PDCCH and a search space of the E-PDCCH when two modes:distributed mode and localized mode are set.

Step (1): Before transmission or reception of a PDSCH, base station 100notifies terminal 200 which may possibly transmit control informationusing an E-PDCCH of the use of the E-PDCCH. Terminal 200 to whichcontrol information is not transmitted on E-PDCCH needs no particularnotification. When receiving no particular notification, terminal 200performs reception while assuming that control information istransmitted on the PDCCH. Terminal 200 which may possibly transmitcontrol information on the E-PDCCH is notified of the settinginformation of a transmission mode which may be possibly used beforetransmission or reception of the PDSCH. For example, both thedistributed mode and localized mode are set for certain terminal 200 andonly one mode, for example, the distributed mode is set for certainterminal 200. Moreover, before transmission or reception of the PDSCH,base station 100 notifies A/N resource candidates determined by the ARIvalue and the transmission mode in which the E-PDCCH is transmitted orreceived. These A/N resource candidates are A to D, and W to Z in FIG.23. These are notified using an RRC control signal or the like.

Step (2): Base station 100 determines terminal 200 to which data isassigned in each subframe and schedules it in the PDSCH. In addition tothe amount of traffic to each terminal 200, CSI feedback or a soundingreference signal (SRS) or the like transmitted by terminal 200 is alsoused for scheduling.

Step (3): Base station 100 generates control information includingscheduling results for each terminal 200 and schedules the PDCCH orE-PDCCH. For terminal 200 for which a plurality of search spaces are seton the E-PDCCH, base station 100 also determines a transmission modeused for transmission.

Base station 100 confirms whether or not collision in A/N resourcesoccurs between all scheduled terminals 200. When collision in A/Nresources occurs, base station 100 changes scheduling results of thePDCCH, ARI value of the E-PDCCH and the transmission mode of the E-PDCCHor the like, and thereby confirms whether or not it is possible to avoidcollision in A/N resources. When collision in A/N resources cannot beavoided, base station 100 gives up the scheduling for terminal 200(allocation block).

Step (4): When the scheduling of control information by all terminals200 is completed, base station 100 transmits, by radio, controlinformation of the PDCCH and E-PDCCH, and downlink data of the PDSCH onthe downlink.

Step (5): Terminal 200 obtains control information intended for terminal200 itself from the received signal and extracts and decodes the datasignal. Terminal 200 to which the control information might have beenpossibly transmitted on the E-PDCCH in particular confirms in whichtransmission mode out of one or a plurality of usable transmission modesthe control information was transmitted. Terminal 200 identifies codeand frequency resources for transmitting A/N signals corresponding tothe received data signal based on the control information. E-PDCCHterminal 200 in particular determines which of A/N resource candidatesnotified beforehand using RRC is used based on the transmission mode inwhich the E-PDCCH intended for terminal 200 itself is transmitted andthe value of the ARI included in the E-PDCCH (e.g., see FIG. 24).

Step (6): Terminal 200 identifies either ACK or NACK in accordance withthe judgment results of the data signal, and transmits the A/N signalusing the A/N resources (code and frequency resources) identified asnoted above.

[Effects]

As described above, base station 100 and terminal 200 according toEmbodiment 4 can increase the number of A/N resource candidates forterminal 200 for which a plurality of E-PDCCH transmission modes are setwithout increasing the number of ARI bits. Furthermore, according toEmbodiment 4, since the number of A/N resource candidates selectable byterminal 200 for which a plurality of transmission modes are set isincreased, it is also possible to reduce the probability that A/Nresources of terminal 200 for which only a single transmission mode, forexample distributed mode, is set will become an allocation block.

Conversely, when the number of A/N resources is greater than the numberof terminals 200 to which the A/N resources are allocated such as whenthe number of terminals 200 is small or when the number of terminals 200allocated on the downlink in the same subframe is small, it is possibleto limit the transmission mode used by limiting the A/N resources to beused to A to D, for example. For example, by limiting the transmissionmode to a distributed mode, E-PDCCHs of all terminals 200 can obtain afrequency diversity effect, and it is thereby possible to achieve highquality E-PDCCH reception. Conversely, by limiting the transmission modeto a localized mode, E-PDCCHs of all terminals 200 are transmitted in asingle PRB, and it is thereby possible to reduce the total number ofPRBs used for the E-PDCCH. It is thereby possible to increase the numberof PRBs usable for the PDSCH and improve the downlink throughput perterminal.

According to Embodiment 4, by additionally setting a transmission modeusable for the E-PDCCH terminal in accordance with the communicationenvironment or terminal situation or the like, it is possible togradually increase the number of A/N resource candidates as required.

According to Embodiment 4, A/N resource candidates, that is, A to D andW to Z in FIG. 23 are all A/N resources notified beforehand by RRCcontrol information or the like. Therefore, base station 100 can easilyadjust A/N resources compared to the allocation method as shown inequation 2 whereby A/N resources are determined by resources such aseCCE numbers for which the E-PDCCH is scheduled. Moreover, this allowsthe circuit scale of base station 100 to be reduced.

[Variation]

In Embodiment 4, when a control signal transmitted on the E-PDCCH usesonly one unit resource such as eCCE, that is, when an aggregation levelis 1, the A/N resource may be determined assuming that the controlsignal is transmitted in the localized mode. In the distributed modehaving a high frequency diversity effect and allowing more terminals 200to perform reception, more terminals are likely to be accommodated.Therefore, when the aggregation level is 1, it is possible to determineA/N assuming that the control signal is transmitted in the localizedmode, and thereby reduce the allocation block probability of the A/Nresources.

Modification Example 1

The communication system of Embodiment 4 can achieve the same effecteven if the following changes are made thereto.

The number of bits of an ARI may be changed depending on thetransmission mode of an E-PDCCH. For example, the number of bits of theARI included in the E-PDCCH transmitted in the distributed mode is 2 andthe number of bits of the ARI included in the E-PDCCH transmitted in thelocalized mode is 1 or the like. Alternatively, the ARI may also be setto have 0 bits depending on the transmission mode. When the ARI has 0bits, one A/N resource given as RRC control information is always used.

By so doing, it is possible to obtain an effect of reducing overhead byreducing the number of ARI bits in addition to effects similar to thoseof Embodiment 4. In an operation in which the E-PDCCH is transmitted tomore terminals in the distributed mode, by adopting more ARI bitsincluded in the E-PDCCH in the distributed mode than in the localizedmode, it is possible to reduce the allocation block rate whilesuppressing the influence of the reduction in the number of ARI bits. Onthe contrary, in an operation in which the E-PDCCH is activelytransmitted in the localized mode, by adopting more ARI bits included inthe E-PDCCH in the localized mode than in the distributed mode, it ispossible to reduce the allocation block rate while suppressing theinfluence of the reduction in the number of ARI bits.

Modification Example 2

The communication system of Embodiment 4 can achieve the same effecteven if the following changes are made thereto.

The range of A/N resource candidates specifiable by the ARI may belimited for each transmission mode in which the E-PDCCH is transmittedor received.

By so doing, it is possible to obtain an effect of reducing overhead ofRRC control signals by limiting the A/N resource candidate settablerange in addition to effects similar to those of Embodiment 4. Forexample, in an operation in which the distributed mode is frequentlyused for the E-PDCCH and the localized mode is used only for fewerE-PDCCH terminals, fewer terminals 200 receive the E-PDCCH in thelocalized mode. Thus, by widening the A/N resource candidate settablerange in the distributed mode and narrowing the A/N resource candidatesettable range in the localized mode, it is possible to reduce theoverhead of RRC while obtaining effects similar to those of Embodiment4.

Embodiment 5

In Embodiment 5, the E-PDCCH is transmitted or received by one of one ora plurality of component carriers (CC) set for a terminal. Here, it isassumed that one or a plurality of CCs used for transmission of theE-PDCCH is/are set for terminal 200 that receives the E-PDCCH. Beforetransmission of a PDSCH, base station 100 notifies terminal 200 of A/Nresources determined by the ARI value and CC whereby the E-PDCCH istransmitted.

In Embodiment 5, base station 100 determines A/N resources to be used byE-PDCCH terminal 200 by the value of an ARI included in an E-PDCCH andCC whereby the E-PDCCH is transmitted. As in the cases of Embodiments 1to 4, terminal 200 selects A/N resources to be used from among A/Nresource candidates notified beforehand based on the value of the ARIincluded in the received E-PDCCH and from which CC the E-PDCCH isdetected. For example, as shown in FIG. 25, two CCs are set totransmit/receive the E-PDCCH and when the E-PDCCH is transmitted fromCC1 or CC2, terminal 200 determines A/N resources to be used based onthe value of the ARI and from which of CC1 or CC2, the E-PDCCH istransmitted.

As described above, base station 100 and terminal 200 according toEmbodiment 5 can increase the number of A/N resource candidates forterminal 200 for which a plurality of CCs are set without increasing thenumber of bits of the ARI. Furthermore, according to Embodiment 5, sincethe number of A/N resource candidates selectable by terminal 200 forwhich the plurality of CCs are set is increased, it is possible toreduce the probability that A/N resources of terminal 200 for which onlya single CC is set will become an allocation block.

According to Embodiment 5, all A/N resources selectable by the ARI andCC are resources notified beforehand by RRC control information or thelike. Therefore, base station 100 can easily adjust A/N resourcescompared to the allocation method as shown in equation 2 whereby A/Nresources are determined by resources such as eCCE number for which theE-PDCCH is scheduled. This allows the circuit scale of base station 100to be reduced.

Embodiment 6

In Embodiment 6, E-PDCCHs are transmitted in one of an interferencecoordination subframe and normal subframe. FIG. 26 illustrates anexample of transmitting or receiving both an interference coordinationsubframe and a normal subframe. In the interference coordinationsubframe, some or all base stations 100 perform transmission with smallpower. Base station 100 notifies beforehand some or all terminals 200 ofa time relationship between the interference coordination subframe andthe normal subframe using RRC control information or the like.

In Embodiment 6, A/N resources used by E-PDCCH terminal 200 aredetermined based on the value of the ARI included in the E-PDCCH andwhether the E-PDCCH is transmitted in the interference coordinationsubframe or the normal subframe. Before transmission of a PDSCH, basestation 100 notifies beforehand terminal 200 of A/N resources determinedbased on the value of the ARI and the type of subframe in which theE-PDCCH is transmitted. As in the cases of Embodiments 1 to 4, terminal200 selects A/N resources to be used from among A/N resource candidatesnotified beforehand based on the value of the ARI included in thereceived E-PDCCH and in which subframe the E-PDCCH is received.

As described above, base station 100 and terminal 200 according toEmbodiment 6 can increase the number of A/N resource candidates withoutincreasing the number of bits of the ARI for terminal 200 to whichinformation on the interference coordination subframe is notified.According to Embodiment 6, since the number of A/N resource candidatesselectable by terminal 200 for which an interference coordinationsubframe is set is increased, it is possible to reduce the probabilitythat A/N resources of terminal 200 for which only normal subframe is setmay become an allocation block.

According to Embodiment 6, all A/N resources selectable based on the ARIand the type of subframe are resources notified beforehand by RRCcontrol information or the like. Therefore, base station 100 can easilyadjust A/N resources compared to the allocation method as shown inequation 2 whereby A/N resources are determined by resources such aseCCE number for which the E-PDCCH is scheduled. This allows the circuitscale of base station 100 to be reduced.

[Variation 1]

A case has been described in Embodiment 6 where, interferencecoordination is performed in subframe units, but even when interferencecoordination is performed in PRB units, similar effects can be achievedusing Embodiment 6. In this case, terminal 200 determines A/N resourcesbased on whether the E-PDCCH is an interference coordination PRB ornormal PRB, and the ARI value.

In this case, in addition to the effects of Embodiment 6, whether or notinterference coordination is performed is set in PRB units, and it isthereby possible to change A/N resources to be used according to the PRBeven in the same subframe.

[Variation 2]

A case has been described in Embodiment 6 whether a subframe is aninterference coordination subframe or normal subframe is used todetermine A/N resources. However, the same effect as that of Embodiment6 can be obtained even when whether a subframe in which the E-PDCCH istransmitted or received is a broadcast type (MBSFN) subframe or a normalsubframe is used to determine A/N resources. Whether a subframe is anMBSFN subframe or not is determined depending on whether a cell-specificreference signal (CRS) is only in a PDCCH time domain or not (e.g., seeFIG. 27). Information on which subframe is an MBSFN subframe is notifiedbeforehand from base station 100 to terminal 200. As in the case ofvariation 1, MBSFN may be a PRB unit instead of subframe unit. If MBSFNis a PRB unit, the number of A/N resource candidates can be increasedwithout increasing the number of bits of the ARI in all subframes.

[Variation 3]

Alternatively, the same effect as that in Embodiment 6 can be obtainedeven when whether a subframe in which the E-PDCCH is transmitted orreceived is a subframe including CRS (w/ CRS) or a subframe notincluding CRS (w/o CRS) is used to determine A/N resources (e.g., seeFIG. 28). As in the cases of variations 1 and 2, whether or not CRS isincluded may be a PRB unit instead of a subframe unit. In the case of aPRB unit, it is possible to increase the number of A/N resourcecandidates without increasing the number of bits of the ARI in allsubframes.

[Variation 4]

Alternatively, the same effect as that in Embodiment 6 can be obtainedeven when whether a subframe in which the E-PDCCH is transmitted orreceived is a subframe assigned semi-static downlink transmission or asubframe assigned downlink transmission by DCI is used to determine A/Nresources. As in the cases of variations 1, 2 and 3, whether semi-staticassignment is applied or not may be determined in PRB units instead ofsubframe units. In the case of the PRB units, it is possible to increasethe number of A/N resource candidates without increasing the number ofbits of the ARI in all subframes.

Note that Embodiments 2 to 6 described so far may be used in combinationin plurality instead of using the respective embodiments separately.Combining the embodiments makes it possible to further increase thedegree of freedom in selecting A/N resources without increasing thenumber of bits of the ARI.

Embodiment 7

[Overview of Communication System]

A communication system according to Embodiment 7 is constructed of oneor a plurality of nodes (macro base station, pico base station), and aplurality of terminals as shown in FIG. 29. A plurality of pico nodesare arranged in a cell of a macro base station forming a large cell(hereinafter, a macro base station in a CoMP scenario 4 is described as“macro node” and a pico base station is described as “pico node”). Thebase station can perform coordinated transmission on the downlink andcoordinated reception on the uplink using the plurality of nodes.

A pico base station may be one such as an RRH (remote radio head). Themacro base station and the pico base stations are connected by alow-delay, high-capacity interface such as an optical fiber, andconstitute a CoMP set. In the following, to avoid having the descriptionbecome complex, constituent elements that are the same as in Embodiment1 are assigned the same reference signs, and only the difference withrespect to Embodiment 1 will be described.

[Configuration of Base Station]

The configuration of base station (macro base station, pico basestations) 100 differs from Embodiment 1 mainly in the difference in theprocessing done by control section 110, with other parts being the sameas Embodiment 1. However, a plurality of base stations 100 are disposedwithin a macrocell, and as described above, these base stations areconnected by a low-delay, high-capacity interface and form a CoMP set.The details of the processing done by control section 110 will bedescribed in detail in the description of operation to follow.

[Configuration of Terminal]

The configuration of terminal 200 differs mainly by the difference inthe processing done by control section 220, with other parts being thesame as in Embodiment 1. The details of the processing done by controlsection 220 will be described in detail in the description of operationto follow.

[Operation]

The processing flow of base station 100 and terminal 200 in Embodiment 7will be described by step (1) through step (6).

Step (1): Before transmission or reception of a PDSCH, base station 100notifies certain terminal 200 which may possibly transmit controlinformation on an E-PDCCH of the use of the E-PDCCH. Terminal 200 towhich control information is not transmitted on E-PDCCH needs noparticular notification. When receiving no particular notification orbeing unable to recognize, terminal 200 receives control informationwhile assuming that the control information is transmitted on the PDCCH.Before transmission or reception of a PDSCH, base station 100 notifiesterminal 200 which may possibly transmit control information on anE-PDCCH of the configuration of the E-PDCCH which may be possibly usedas RRC control information. Furthermore, before transmission orreception of the PDSCH, base station 100 notifies terminal 200 of A/Nresource candidates determined by the ARI value and the configuration ofthe E-PDCCH as RRC control information. These A/N resource candidatesare A to D, and W to Z in FIG. 30. Moreover, base station 100 notifiesterminal 200 of a virtual cell ID corresponding to each A/N resource asRRC control information. Here, the virtual cell ID refers to a basesequence number necessary to transmit a PUCCH, a sequence hoppingpattern or an ID necessary to determine a cyclic shift (CS) hoppingpattern. In a conventional system in Rel.10, this ID is a cell ID and aparameter common among all terminals 200 in a cell, but in presentEmbodiment 7, the virtual cell ID is assumed to be a parameterindividually settable for terminal 200.

Step (2): Base station 100 determines terminal 200 to which data isassigned in each subframe and schedules it in the PDSCH. In addition tothe amount of traffic to each terminal 200, CSI feedback or soundingreference signal (SRS) or the like transmitted by terminal 200 is alsoused for scheduling.

Step (3): Base station 100 generates control information includingscheduling results for each terminal 200 and schedules the PDCCH andE-PDCCH. Base station 100 also determines the configuration used forE-PDCCH transmission for terminal 200 in which a plurality of E-PDCCHconfigurations are set.

Base station 100 confirms whether or not collision in A/N resourcesoccurs between all scheduled terminals 200. When collision in A/Nresources occurs, base station 100 changes the scheduling results of thePDCCH, ARI value of the E-PDCCH and the configuration of the E-PDCCH orthe like, and thereby confirms whether or not it is possible to avoidcollision in A/N resources. When collision in A/N resources cannot beavoided, base station 100 gives up the scheduling for terminal 200(allocation block).

Step (4): When the mapping of control information by all terminals 200is completed, base station 100 transmits, by radio, control informationof the PDCCH and E-PDCCH, and downlink data of the PDSCH on thedownlink.

Step (5): Terminal 200 obtains control information intended for terminal200 itself from the received signal and extracts and decodes the datasignal. Terminal 200 to which the control information might have beenpossibly transmitted on the E-PDCCH in particular also confirms in whichconfiguration out of one or a plurality of usable configurations thecontrol information was transmitted. Terminal 200 identifies code andfrequency resources for transmitting A/N signals corresponding to thereceived data signal based on the control information. E-PDCCH terminal200 in particular determines which of A/N resource candidates notifiedbeforehand using RRC is used based on the configuration of the E-PDCCHintended for terminal 200 itself and the value of the ARI included inthe E-PDCCH. Terminal 200 also determines a virtual cell IDcorresponding to the A/N resource. FIG. 30 illustrates an example of A/Nresources and corresponding virtual cell IDs determined by the E-PDCCHconfiguration and the ARI. In FIG. 30, although VCID-0 and VCID-1 areset as virtual cell IDs, different virtual cell IDs may also be set forall A/N resources.

Step (6): Terminal 200 identifies either ACK or NACK in accordance withthe judgment results of the data signal, and transmits the A/N signalusing the A/N resources (code and frequency resources) identified asnoted above. A/N signals are transmitted by the PUCCH. Terminal 200generates a base sequence number, base sequence hopping pattern, and CShopping pattern of the PUCCH using virtual cell IDs corresponding to theabove-described A/N resources.

[Effects]

In addition to the effects of Embodiment 1, Embodiment 7 can generate aPUCCH with different virtual cell IDs according to the configuration ofthe E-PDCCH. A/N resources of the PUCCH can be orthogonalized onlybetween terminals 200 having an identical virtual cell ID or cell ID.Therefore, according to Embodiment 7, it is possible to transmit A/Nsignals that can be multiplexed with a terminal 200 group which differsfrom one E-PDCCH configuration to another. This allows, for example,terminal 200 located between a macro base station and a pico basestation to generate A/N signals that are received at the macro basestation and can be multiplexed with signals from a macro terminal, andA/N signals that are received at the pico base station and can bemultiplexed with signals from a pico terminal according to the E-PDCCHconfiguration.

When the E-PDCCH configuration supports different transmission basestations, Embodiment 7 allows the ID to be switched to a virtual cell IDused to receive A/N signals at an E-PDCCH transmitting station. Since anA/N signal is a signal used for retransmission control, receiving an A/Nsignal at the E-PDCCH transmitting base station makes it possible toperform retransmission control with reduced delay or backhaul burden.

Not only a virtual cell ID but also parameters such as transmissionpower or timing offset may be set so as to be dynamically switchedaccording to the E-PDCCH configuration. In this way, even when thedistance from terminal 200 to the macro base station is considerablydifferent from the distance from terminal 200 to the pico base station,it is possible to receive A/N signals by switching between A/N signalreceiving stations.

In Embodiment 7, the number of virtual cell IDs is assumed to be 2, butone ID may be a cell-specific ID (cell ID). This is because one of theE-PDCCH transmitting base stations is very likely to be a cell connectedto the terminal on the downlink. Employing a known cell ID for one ofthe virtual cell IDs makes it possible to reduce overhead of an RRCcontrol signal while achieving effects comparable to those of Embodiment7.

The embodiments of the present invention have been described thus far.

In the embodiments described above, the present invention is describedusing an example of a case where the present invention is implemented ashardware. However, the present invention can be achieved by software inconcert with hardware.

The functional blocks described in the embodiments described above areachieved by an LSI, which is typically an integrated circuit. Thefunctional blocks may be provided as individual chips, or part or all ofthe functional blocks may be provided as a single chip. Depending on thelevel of integration, the LSI may be referred to as an IC, a system LSI,a super LSI, or an ultra LSI.

In addition, the circuit integration is not limited to LSI and may beachieved by dedicated circuitry or a general-purpose processor otherthan an LSI. After fabrication of LSI, a field programmable gate array(FPGA), which is programmable, or a reconfigurable processor whichallows reconfiguration of connections and settings of circuit cells inLSI may be used.

Should a circuit integration technology replacing LSI appear as a resultof advancements in semiconductor technology or other technologiesderived from the technology, the functional blocks could be integratedusing such a technology. Another possibility is the application ofbiotechnology and/or the like.

A radio communication terminal according to the above-describedembodiment adopts a configuration including a receiving section thatreceives a control signal including an ACK/NACK index via an enhancedphysical downlink control channel (E-PDCCH) transmitted using oneconfiguration from among one or a plurality of configuration candidates,a control section that selects a resource to be used for an ACK/NACKsignal of downlink data from among specified resources specifiedbeforehand based on E-PDCCH configuration information used fortransmission or reception of the E-PDCCH and the ACK/NACK index, and atransmitting section that transmits the ACK/NACK signal using theselected specified resource.

This configuration allows the number of A/N resource candidates toincrease for terminal 200 for which a plurality of E-PDCCHconfigurations are set beforehand without increasing the number of bitsof the ARI. This makes it possible to reduce the probability ofallocation block also for terminal 200 for which only a single E-PDCCHconfiguration is set. By adding a configuration usable for an E-PDCCHterminal in accordance with a communication environment or terminalsituation or the like, it is possible to gradually increase the numberof A/N resource candidates as required.

In the radio communication terminal according to the above-describedembodiment, the ACK/NACK index indicates a number of states that differsdepending on the configuration of the E-PDCCH transmitted and thecontrol section selects a resource to be used for an ACK/NACK signal ofdownlink data from among the specified resources specified beforehandbased on the configuration information used for transmission orreception of the E-PDCCH and the ACK/NACK index.

This makes it possible to reduce the number of bits required to selectACK/NACK resources according to an ACK/NACK index and reduce overhead.

In the radio communication terminal according to the above-describedembodiment, the specified resources are to be specified within a rangethat differs depending on the configuration used for transmission orreception of the E-PDCCH and the control section selects a resource tobe used for an ACK/NACK signal of downlink data from among the specifiedresources specified beforehand, based on the configuration informationused for transmission or reception of the E-PDCCH and the ACK/NACKindex.

Thus, by limiting the range in which resources can be used as ACK/NACKresources, it is possible to narrow the range of values that can betaken by the ACK/NACK resources and reduce overhead required to notifythe specified resources.

In the radio communication terminal according to the above-describedembodiment, the configuration candidates are each a frequency resourceblock (PRB) set.

Thus, it is possible to increase the number of specified resources forterminal 200 for which a plurality of E-PDCCH PRB sets are setbeforehand without increasing the number of states of the ACK/NACKindex. Moreover, by adding PRB sets that can be used for an E-PDCCHterminal in accordance with the communication environment or terminalsituation or the like, it is possible to gradually increase the numberof A/N resource candidates as required. Furthermore, since all theincreased specified resources are ACK/NACK resources notified beforehandfrom the base station, it is easy for the base station to adjustACK/NACK resources between terminals and the base station can beoperated with a small circuit scale or algorithm.

In the radio communication terminal according to the above-describedembodiment, when PRBs in which the control signal including the ACK/NACKindex is transmitted or received are PRBs belonging to two or more PRBsets, the control section assumes that the PRBs belong to apredetermined specific PRB set and selects an ACK/NACK resource fromamong the specified resources specified beforehand using the specificPRB set and the ACK/NACK index.

Even when it is not possible to distinguish a PRB set of PRBs throughwhich a control signal including an ACK/NACK index is transmitted orreceived, ACK/NACK resources can be uniquely determined, and the basestation thereby no longer needs to reserve ACK/NACK resourcescorresponding to a plurality of PRB sets and ACK/NACK resourceutilization efficiency improves.

In the radio communication terminal according to the above-describedembodiment, when PRBs in which the control signal including the ACK/NACKindex is transmitted or received are PRBs belonging to two or more PRBsets, the control section assumes that the PRBs belong to a specific PRBset in which an interval between a PRB having a lowest frequency and aPRB having a highest frequency is small, and selects an ACK/NACKresource from among the specified resources specified beforehand usingthe specific PRB set and the ACK/NACK index.

Since ACK/NACK resources are determined assuming that the PRBs belong toa PRB set having a small number of terminals, the ACK/NACK is lesslikely to be used by other terminals and it is possible to reduce theprobability of ACK/NACK resources becoming an allocation block.

In the radio communication terminal according to the above-describedembodiment, the configuration candidates are each search space.

Thus, it is possible to increase the number of specified resources forterminal 200 for which a plurality of search spaces are set beforehandwithout increasing the number of states of the ACK/NACK index. Moreover,by adding a search space that can be used for an E-PDCCH terminal inaccordance with the communication environment or terminal situation orthe like, it is possible to gradually increase the number of A/Nresource candidates as required. Furthermore, since all the increasedspecified resources are ACK/NACK resources notified beforehand from thebase station, it is easy for the base station to adjust ACK/NACKresources between terminals and the base station can be operated with asmall circuit scale or algorithm.

In the radio communication terminal according to the above-describedembodiment, when PRBs in which the control signal including the ACK/NACKindex is transmitted or received are PRBs belonging to two or moresearch spaces, the control section assumes that the PRBs belong to aterminal-specific search space (USS) and selects an ACK/NACK resourcefrom among the specified resources specified beforehand using the USSand the ACK/NACK index.

Even when it is not possible to distinguish a search space through whicha control signal including an ACK/NACK index is transmitted or received,ACK/NACK resources can be uniquely determined, and the base stationthereby no longer needs to reserve ACK/NACK resources corresponding to aplurality of search spaces and ACK/NACK resource utilization efficiencyimproves. Moreover, using ACK/NACK resources corresponding to a USSwhich is likely to have fewer simultaneously accommodated terminalsmakes it possible to reduce the probability of the ACK/NACK resourcesbecoming an allocation block.

In the radio communication terminal according to the above-describedembodiment, the configuration candidates are each a transmission modeused for transmission of an E-PDCCH and the transmission mode is adistributed mode in which the E-PDCCH is transmitted using two or morePRBs or a localized mode in which the E-PDCCH is transmitted using onlyone PRB.

It is thereby possible to increase the number of specified resourceswithout increasing the number of states of the ACK/NACK index forterminal 200 for which both a distributed mode and a localized mode areset beforehand. Moreover, by adding a transmission mode usable for anE-PDCCH terminal in accordance with the communication environment orterminal situation or the like, it is possible to gradually increase thenumber of A/N resource candidates as required. Furthermore, since allthe increased specified resources are ACK/NACK resources notifiedbeforehand from the base station, it is easy for the base station toadjust ACK/NACK resources between terminals and the base station can beoperated with a small circuit scale or algorithm.

In the radio communication terminal according to the above-describedembodiment, the configuration candidates are each a component carrier(CC).

It is thereby possible to increase the number of specified resourceswithout increasing the number of states of the ACK/NACK index forterminal 200 for which the use of a plurality of CCs is set beforehand.Moreover by adding CCs usable for an E-PDCCH terminal in accordance withthe communication environment or terminal situation or the like, it ispossible to gradually increase the number of A/N resource candidates asrequired. Furthermore, since all the increased specified resources areACK/NACK resources notified beforehand from the base station, it is easyfor the base station to adjust ACK/NACK resources between terminals andthe base station can be operated with a small circuit scale oralgorithm.

In the radio communication terminal according to the above-describedembodiment, the control section selects a resource to be used for anACK/NACK signal of downlink data, a base sequence of a PUCCH fortransmitting the ACK/NACK signal, and a virtual cell ID to be used togenerate a hopping pattern or cyclic shift (CS) hopping pattern fromamong the specified resources specified and specified virtual cell IDsbeforehand, based on the configuration information of the E-PDCCH usedfor transmission or reception of the E-PDCCH and the ACK/NACK index, andthe transmitting section transmits the ACK/NACK signal using theselected specified resource and specified virtual cell ID.

It is thereby possible to increase the number of A/N resource candidateswithout increasing the number of bits of the ARI for terminal 200 forwhich a plurality of E-PDCCH configurations are set beforehand.Furthermore, by changing the format of ACK/NACK signals to betransmitted in accordance with the configuration and the state of theACK/NACK index, it is possible to multiplex a plurality of differentterminal groups with the ACK/NACK signals and transmit the multiplexedsignal.

A base station apparatus according to the above-described embodimentadopts a configuration including a control section that determines aresource for transmitting an ACK/NACK signal in response to downlinkdata from a radio communication terminal from among specified resourcesspecified beforehand, based on a configuration used for transmission ofan E-PDCCH out of one or a plurality of E-PDCCH configurations indicatedbeforehand to the radio communication terminal and an ACK/NACK indexincluded in a control signal and a transmitting section that transmitsthe control signal including the ACK/NACK index indicating thedetermination result of the control section via the E-PDCCH using theconfiguration corresponding to the determined specified resource.

This makes it possible to switch between specified resources of anE-PDCCH terminal specifiable by the ACK/NACK index according to theconfiguration of the E-PDCCH transmitted. It is thereby possible toincrease the number of alternatives in selecting ACK/NACK withoutincreasing the number of states of the ACK/NACK index.

In the base station apparatus according to the above-describedembodiment, the configurations are each a frequency resource block (PRB)set.

Thus, specified resources of an E-PDCCH terminal specifiable by theACK/NACK index can be switched by a PRB set of the E-PDCCH transmitted.It is thereby possible to increase the number of alternatives inselecting ACK/NACK without increasing the number of states of theACK/NACK index.

In the base station apparatus according to the above-describedembodiment, the configurations are each a search space.

Thus, specified resources of an E-PDCCH terminal specifiable by theACK/NACK index can be switched by a search space of the E-PDCCHtransmitted. It is thereby possible to increase the number ofalternatives in selecting ACK/NACK without increasing the number ofstates of the ACK/NACK index.

In the base station apparatus according to the above-describedembodiment, the configurations are each a transmission mode used fortransmission of an E-PDCCH and the transmission mode is a distributedmode in which the E-PDCCH is transmitted using two or more PRBs or alocalized mode in which the E-PDCCH is transmitted using only one PRB.

Thus, specified resources of an E-PDCCH terminal specifiable by theACK/NACK index can be switched according to a transmission mode of theE-PDCCH transmitted. It is thereby possible to increase the number ofalternatives in selecting ACK/NACK without increasing the number ofstates of the ACK/NACK index.

In the base station apparatus according to the above-describedembodiment, the configurations are each a component carrier (CC).

It is thereby possible to switch between specified resources of anE-PDCCH terminal specifiable by the ACK/NACK index in accordance withthe CC by which the E-PDCCH is transmitted. It is thereby possible toincrease the number of alternatives in selecting ACK/NACK withoutincreasing the number of states of the ACK/NACK index.

In the base station apparatus according to the above-describedembodiment, the control section determines a resource for transmittingan ACK/NACK signal in response to downlink data from a radiocommunication terminal and a virtual cell ID for generating a PUCCH fromamong the specified resources specified and specified virtual cell IDsbeforehand, based on the configuration used for transmission of theE-PDCCH out of one or a plurality of E-PDCCH configurations indicatedbeforehand to the radio communication terminal and the ACK/NACK indexincluded in the control signal, and the transmitting section transmitsthe control signal including the ACK/NACK index indicating thedetermination result of the control section via the E-PDCCH using theconfiguration corresponding to the determined specified resource andspecified virtual cell ID.

This makes it possible to switch between specified resources of anE-PDCCH terminal specifiable by the ACK/NACK index according to theconfiguration in which the E-PDCCH is transmitted. It is therebypossible to increase the number of alternatives in selecting ACK/NACKwithout increasing the number of states of the ACK/NACK index. Moreover,by switching a virtual cell ID for generating a PUCCH according to theconfiguration in which the E-PDCCH is transmitted, it is possible toswitch between terminal groups in which ACK/NACK signals transmitted bythe PUCCH can be multiplexed.

A resource allocation method according to the above-described embodimentincludes receiving a control signal including an ACK/NACK index via anenhanced physical downlink control channel (E-PDCCH) and selecting oneof resource candidates specified beforehand from among a plurality ofACK/NACK resources separated from each other in frequency and coderegions based on the ACK/NACK index and a configuration of the E-PDCCH.

This makes it possible to switch between specified resources of anE-PDCCH terminal specifiable by the ACK/NACK index according to theconfiguration of the E-PDCCH transmitted. It is thereby possible toincrease the number of alternatives in selecting ACK/NACK withoutincreasing the number of states of the ACK/NACK index.

In the resource allocation method according to the above-describedembodiment, the configurations are each a frequency resource block (PRB)set.

Thus, the specified resources of the E-PDCCH terminal specifiable by theACK/NACK index can be switched by a PRB set whereby the E-PDCCH istransmitted or received. It is thereby possible to increase the numberof alternatives in selecting ACK/NACK without increasing the number ofstates of the ACK/NACK index.

In the resource allocation method according to the above-describedembodiment, the configurations are each a search space.

Thus, the specified resources of the E-PDCCH terminal specifiable by theACK/NACK index can be switched by a search space whereby the E-PDCCH istransmitted. It is thereby possible to increase the number ofalternatives in selecting ACK/NACK without increasing the number ofstates of the ACK/NACK index.

In the resource allocation method according to the above-describedembodiment, the configurations are each a transmission mode used fortransmission or reception of an E-PDCCH and the transmission mode is adistributed mode in which the E-PDCCH is transmitted using two or morePRBs or a localized mode in which the E-PDCCH is transmitted using onlyone PRB.

Thus, the specified resources of the E-PDCCH terminal specifiable by theACK/NACK index can be switched according to a transmission mode in whichthe E-PDCCH is transmitted. It is thereby possible to increase thenumber of alternatives in selecting ACK/NACK without increasing thenumber of states of the ACK/NACK index.

In the resource allocation method according to the above-describedembodiment, the configurations are each a component carrier (CC).

It is thereby possible to switch between the specified resources of theE-PDCCH terminal specifiable by the ACK/NACK index in accordance withthe CC by which the E-PDCCH is transmitted. It is thereby possible toincrease the number of alternatives in selecting ACK/NACK withoutincreasing the number of states of the ACK/NACK index.

In the resource allocation method according to the above-describedembodiment, a virtual cell ID is further selected which is necessary togenerate a base sequence, hopping pattern or cyclic shift hoppingpattern of a PUCCH for transmitting an ACK/NACK signal based on theACK/NACK index and the configuration used for transmission or receptionof the E-PDCCH.

This makes it possible to switch between the specified resources of theE-PDCCH terminal specifiable by the ACK/NACK index according to theconfiguration in which the E-PDCCH is transmitted. It is therebypossible to increase the number of alternatives in selecting ACK/NACKwithout increasing the number of states of the ACK/NACK index. Byswitching the virtual cell ID for generating the PUCCH in accordancewith the configuration in which the E-PDCCH is transmitted, it ispossible to switch between terminal groups in which ACK/NACK signalstransmitted by the PUCCH can be multiplexed.

The disclosure of Japanese Patent Application No. 2012-172224, filed onAug. 2, 2012, including the specification, drawings and abstract isincorporated herein by reference in its entirety.

INDUSTRIAL APPLICABILITY

The present invention can be applied to a radio communication terminal,a base station apparatus, a resource allocation method and the like of amobile communication system.

REFERENCE SIGNS LIST

11 Antenna

12 Control information generation section

13 Control information coding section

14, 17 Modulation section

15 Data coding section

16 Retransmission control section

18 Subframe configuration section

19 IFFT section

20 CP appending section

21 Radio transmitting section

22 Radio receiving section

23 CP removal section

24 Despreading section

25 Correlation processing section

26 Judgment section

41 Antenna

42 Radio receiving section

43 CP removal section

44 FFT section

45 Extraction section

46 Data demodulation section

47 Data decoding section

48 Judgment section

49 Control information demodulation section

50 Control information decoding section

51 Control information judgment section

52 Control processing section

53 A/N signal modulation section

54 Primary spreading section

55, 60 IFFT section

56 CP appending section

57 Secondary spreading section

58 Multiplexing section

59 Radio transmitting section

61 CP appending section

62 Spreading section

100 Base station

110 Control section

120 Transmitting section

200 Terminal

210 Transmitting section

220 Control section

230 Receiving section

The invention claimed is:
 1. A base station comprising: a transmitter,which, in operation, transmits an enhanced physical downlink controlchannel (EPDCCH) including an ACK/NACK Resource Indicator (ARI) in oneor more enhanced control channel elements (eCCEs) of a physical resourceblock (PRB) set; and a receiver, which, in operation, receives anACK/NACK signal using an ACK/NACK resource; wherein the ACK/NACKresource depends on both the PRB set and a value of the ARI, the ARIbeing 2 bits and having a value independent of the eCCEs.
 2. The basestation according to claim 1, wherein one or more PRB sets including thePRB set are configured.
 3. The base station according to claim 2,wherein all of the one or more PRB sets have the same PRB frequencyinterval.
 4. The base station according to claim 2, wherein the one ormore PRB sets respectively have different PRB frequency intervals. 5.The base station according to claim 2, wherein the ARI is correlatedwith the one or more PRB sets.
 6. The base station according to claim 2,wherein the ARI is included in one of one or more ARI sets which arecorrelated with the one or more PRB sets.
 7. The base station accordingto claim 1, wherein the transmitter, in operation, notifies a terminalapparatus of one or more PRB sets including the PRB set.
 8. The basestation according to claim 7, wherein the ARI is correlated with the oneor more PRB sets.
 9. The base station according to claim 1, wherein thePRB set includes one or more PRBs.
 10. The base station according toclaim 1, wherein the ACK/NACK resource depends on a transmission modeused to transmit the EPDCCH, and the transmission mode is either adistributed mode where the EPDCCH is transmitted using two or more PRBsor a localized mode where the EPDCCH is transmitted using only one PRB.11. A communication method implemented by a base station and comprising:transmitting an enhanced physical downlink control channel (EPDCCH)including an ACK/NACK Resource Indicator (ARI) in one or more enhancedcontrol channel elements (eCCEs) of a physical resource block (PRB) set;and receiving an ACK/NACK signal using an ACK/NACK resource; wherein theACK/NACK resource depends on both the PRB set and a value of the ARI,the ARI being 2 bits and having a value independent of the eCCEs. 12.The communication method according to claim 11, wherein one or more PRBsets including the PRB set are configured.
 13. The communication methodaccording to claim 12, wherein all of the one or more PRB sets have thesame PRB frequency interval.
 14. The communication method according toclaim 12, wherein the one or more PRB sets respectively have differentPRB frequency intervals.
 15. The communication method according to claim12, wherein the ARI is correlated with the one or more PRB sets.
 16. Thecommunication method according to claim 12, wherein the ARI is includedin one of one or more ARI sets which are correlated with the one or morePRB sets.
 17. The communication method according to claim 11,comprising: notifying a terminal apparatus of one or more PRB setsincluding the PRB set.
 18. The communication method according to claims17, wherein the ARI is correlated with the one or more PRB sets.
 19. Thecommunication method according to claim 11, wherein the PRB set includesone or more PRBs.
 20. The communication method according to claim 11,wherein the ACK/NACK resource depends on a transmission mode used totransmit the EPDCCH, and the transmission mode is either a distributedmode where the EPDCCH is transmitted using two or more PRBs or alocalized mode where the EPDCCH is transmitted using only one PRB.